OP I think your first post was badly explained and so it is perhaps going to effect the replies you get going forward. I was also ready to tell you not to interfere until your follow up.
It sounds like the issue is more that your DD did not seem satisfied with anything you said.
She said she wanted your advice but I wonder if maybe she actually just wanted to talk about how unfair it all was and how angry and confused she is?
It's a difficult one. She's displaying healthy boundaries and as far as I'm concerned, ending the friendship is probably the best idea. Someone telling her to forgive probably does make her boundaries feel trampled and like she's expected to put up with being hurt.
At the same time, losing a friend hurts and she perhaps is feeling guilty / angry / sad that the friendship is over. So someone saying "just stop being her friend you're better off without her" would perhaps also not satisfy her and feel invalidating.
Basically, when it comes to giving advice it's often the case that there's no right thing to say.
The confusion and conflicting feelings are likely prompting her to explicitly ask for advice but getting advice isn't helping because she already knows her options and frankly they both feel a bit shit.
My general advice would be to leave her to it, but if she does come to you with this, I wonder if you could try and avoid jumping in with the advice and instead focus on empathising and validating her feelings and see if she responds better to that.
"I don't know whether to forgive her. She says she's sorry but what she did wasn't right."
"It does sound like a difficult decision. Laura has been your friend for this whole time and you've had lots of good memories together but you also want to make sure you aren't treated badly by people."
"What shall I do?"
"I wish I had a simple answer that would feel easy. What are the different parts of you saying?"
Etc etc.
There is definitely a place for you sharing what you personally would do, with the caveat that everyone is different and she has to decide herself. But I wonder if perhaps just some slight tweaks to your phrasing and taking a step back to try and help her to get to her own decision first would feel better and make her less defensive.