I think you do whatever you need to do in order to move forward. If not forgiving someone means they then take up residence in your head and you ruminate all the time about why they were so (whatever they were) then it is in your best interests to figure out how to let that go. Forgiveness tends to imply that you resume some prior state of being with them, such as your friend resuming a good relationship with her husband, but I don't think that is necessary for the kind of forgiveness that I think is being talked about here.

I had a spiritual teacher who got his ego bruised and then ripped me to shreds in front of all of my peers in a very insensitive and humiliating way. I don't want anything more to do with him. I'll never think he is a good person. But I do forgive him so that he has absolutely no more control over my life. For me, the reason to forgive someone is so that they have no more power over your feelings. Holding onto the resentment of what this other women did just keeps her alive in your friend's head. But if she can truly move on without letting it go in that way, more power to her.