ABC I think Phee has been remarkably restrained, in her situation. She's had a horrible shock and been really badly betrayed and hurt by a man who is supposed to love her.
I agree that more responsibility lies with her H. I also agree that yes, occasionally you can fall in love with someone else and it can't be helped and you do leave and it does work out. And of course that's also a horrible shock and very hurtful.
The differentiator on this occasion, IMO, is the horrible way Phee's H and the OW have acted. They didn't fall for each other, refuse to act on feelings, stay away from each other and give H time to fix his marriage. They simply went behind Phee's back. Since she found out and he left, her H has behaved appallingly: no apologies, no remorse, no respect for how much she's been hurt, daring to give her parenting tips from the OW. The OW is also not behaving well for much the same reason.
I did not have DC but I was once in a LTR where I fell for someone else. I can tell you I agonised about it for months. Nothing happened with the OM and I kept away from him, went NC as felt I owed it to my partner to try to fix it with him. Eventually realised I was just unhappy in the relationship and would have been OM or no OM - he was just the catalyst that made me realise it. So I broke up with partner. Left it a good 6 months before OM and I became an item, did not go out anywhere my ex was likely to be, and certainly did not take selfies or post happy little Facebook statuses two days after I had broken his heart.
OW had a choice in this situation and so did Phee's H. I judge them both on their actions rather than their feelings. If her H was genuinely unhappy he owed it to Phee to be honest with her and to try and work through it, not to shag OW behind her back!!
And breathe.