AIBU to expect an apology after my friend crossed boundaries with my husband?

Lovely, it will never be enough. An apology will not fix what has happened, nor your suspicions in the future if you choose to continue this relationship. I have been brutally honest because I have been there. What actual difference would her saying sorry make - honestly? Nothing really - if you are being honest.

She would be dead to me so I wouldn’t give a shit about her apology anyway. My husband would also be dead to me btw!

I think it's highly unlikely that she's going to apologise so you'll have to find a way to make your peace with that.

Yup, this. Forget her, the friendship is over (and should be, trust is gone). Bigger issue is your DH and your marriage. He is of course playing down his role in this. You need to think about why he did this, and whether you can trust him again, and whether you want to. His behaviour was as bad as hers, worse actually given he’s the one married to you. I couldn’t, and wouldn’t get over this, his betrayal and with such a close friend is unforgivable. However if you feel you can, I would suggest couples counselling. He needs to acknowledge his behaviour and the impact, and why he did this.

When it happened to me the LAST thing I wanted from her was an apology, I wanted her never to approach nor to speak to me again. I wanted her SO FAR AWAY FROM ME that we weren't in the same atmosphere. It's a dumb thing to want, because even if it happens the rot of your falling apart marriage is still there

you want closure she doesnt care you wont get it from her

Both your husband and her were equally wrong in the first place-him because he presumably made marriage vows, her because it is just as wrong for a best friend to betray you as a husband. I'm not going with the idea of a woman breaking "girl code" as being worse, because we shouldn't expect women to be better than men.
But she is worse NOW if she's carrying on-and maybe to the point of stalking, if he (really) has told her to back off and she won't-if she really is harassing him, he should report it.

Because you asked “why would she apologise”? I think that this rancid woman does owe op an apology. I can think of precisely zero circumstances under which I would try it on with a friends husband but if I had? Damn right I would apologise!
And I don’t hold with this “I made no vows” bollocks that OW/OM use to justify their shitty behaviour, common decency isn’t difficult, but yes, the married party is usually “more” to blame. But when it’s your friend? Nope. Both cunts.

Block her completely and cut her out of your life, that’s your closure. No apology or anything she says will make you feel better.

Focus your attention on deciding whether your DH is worth the effort.

I mean, this. An apology when someone isn’t sorry is less than worthless, especially when you know she’d do it again in a heartbeat and she’s not sorry at all. She couldn’t give a fuck about losing your friendship. She was worried about all the men: she wanted your husband AND THEN she didn’t want you to tell her husband. Also, the research is in: those who post excessively over-the-top lovey-dovey shite on Facebook are almost always engaging in relationship overcompensation, and they have less satisfaction and stability in their relationship. Psychology Today cites the studies:
www.psychologytoday.com/us/blog/close-encounters/201811/is-social-media-pda-sign-happiness-or-overcompensation#

She is not a friend OP.
She has ruined any hope of being a part of your life after her despicable behaviour.
You need to focus on whether you're able to forgive your husband and move forwards after his dishonesty too.

Personally I wouldnt want an apology from someone who had behaved like that. It would be pointless, I would never trust them again or want them in my life and apology would just be one more thing for me to have to process. I would also assume by apologising they were expecting forgivenesss eventually and that would not be happening. Focus on your marriage and figuring out what happened there, maybe some counselling to get to the bottom of whatever unspoken feelings he was sitting on in the marriage that he hadn't voiced that may have caused him to cross a line. Im not saying you should shoulder any blame, not at all, but there are alway reasons for people allowing this kind of thing to happen and its worth it to find out what they are, and sort it out. The relationship will be stronger as a result. And forget about her, block, and delete.

Why are you getting on at me just because I mentioned her husband is more to blame - and he is by the way! I had the same situation with a friend problem also, but guess what, I focused on the person who had made the vows! I also had a double shit sandwich! And the OP seems to be blithely blaming only one person and it's not her shitty husband!!!! Re-read the thread title - she wants an apology from the other woman - what a joke.

If she didn't respect you enough to stay away from your husband, of course she doesn't respect you enough to apologise.

It would be meaningless anyway.

Describing him as her life etc. seems very extreme for just a flirty exchange, you sure it never went further?

As for your husband, why even try to repair things with a man who not only betrayed you but did so with your best friend. I couldn't come back from that.

You’re asking the wrong source.

When you chase closure from another person, you’re basically saying ‘explain this in a way that makes my pain go away’. That’s a tall order, and usually impossible.

She likely doesn’t fully understand her own behaviour, would avoid honesty to protect herself or give you answers that raise more questions.

There’s rarely a clean, satisfying answer or an apology you could accept. The answers or apology will feel incomplete. Your brain will keep searching for a deeper meaning that simply isn’t there, because your brain wants certainty, not truth. You think if you just understand it fully, you’ll feel better. In reality, understanding and relief are not the same thing.

Closure is something you create, not receive. It isn’t an explanation, an apology or a final conversation. Instead focus on acceptance over explanation, boundaries and what YOU have learnt.

OP, I'm so sorry.

I wouldn't give her a second thought. My marriage, on the other hand...

Eh? My jets are perfectly cool thanks, but I simply cannot imagine ever doing this to someone I consider a “friend”. Or even someone I don’t, for that matter.
For everyone saying op has a husband problem, obviously she absolutely does. But the poor woman has both a husband AND a friend problem. That sounds like a double shit sandwich for op and whilst the husband is clear horrendous, the shitty “friend” is also not blameless. Vile behaviour from both of them.

An apology would be absolutely meaningless from a marriage wrecking bitch like that. What she deserves is to be cut off and ghosted by you until the end of time.
If she was sorry she wouldnt have started this.
Id dump him as well.

So he says. He only fessed up when he was caught.

I think if your closure requires an apology you will be struggling for a long time - sorry. I've been there and it's horrible.

Can I suggest that perhaps it's more about struggling to make the transition in your head of her as a friend into her as an untrustworthy villain in your story. It takes a while to catch up mentally and reframe that person as someone who doesn't care about you. It's especially hard as you're having to put them both in separate categories (her, betrayed you and no longer worth your time. Him, betrayed you but possibly worth figuring things out with) and that screws with your head in an extra special sort of way. Take your time to process and have no expectations of ever seeing her or receiving any apologies or closure. I would actively avoid any contact with them now. She's living in a mess of her own making and clearly has no bandwidth to consider your feelings.

Take.your time with your husband and don't let anyone hurry or pressure you about making decisions.

Why are doing the work to fix your marriage? This is his shit to fix.

Other than apologise, is he doing the hard work needed to try and fix the broken trust?

You’re right, but he did close it down and she repeatedly tried to restart it.