You and Joshy both...
That's fine and dandy, it's okay to like her, I don't think less of you or anyone else. You just won't ever catch me simping that hard. I can already anticipate that she was forced to do it under duress, via blackmail, coercion, or some other method. However, despite that, I'm not forgiving her, I can learn to co-exist with her if Mary wants to remain her friend, because that's Mary's decision. This doesn't mean however, that I want any sort of friendly/sexual relationship with her, or that I'll ever trust her again.
If she was going after something else, like the MC's job for example, then I could forgive her, because a job is easily replaceable anyways. In that instance, it's understandable to a degree, and at the end of the day no harm done. She was going after his marriage though, which is something that isn't easily replaceable, and by doing so also betraying her best friend. I don't care the reason, you ain't ever getting my forgiveness after that, you can have my empathy, I won't hate you for it, but you aren't my friend.