I’m going to offer a slightly different perspective here.
I don’t think it’s quite as simple as just leaving her husband.I assume she has always identified as straight - so she will have to come out to her family, friends, kids?, work colleagues etc. That’s pretty daunting. People who aren’t married struggle to come out for years, let alone if you have to change your entire life and that of her husbands.
Cheating is awful, but your friend is just as bad IMO, she knows she’s married. However, I don’t think it’s as simple in this case as she’s not leaving him because she doesn’t want to so she never will. She was probably experimenting with your friend and is now coming to terms with her sexuality, that doesn’t happen in a matter of weeks.