@liliputian1 fuck her best friend 😂
On a serious note, you don't. Cheating/betrayals are not accidental.
It's a choice that someone made knowing full well it would hurt you. They just didn't care enough. Their temporary happiness/pleasure was more important to them than your trust/well-being. If you forgive someone for something like this, you are saying this kind of shit is acceptable to you, as long as they feel bad and don't do it again. They didn't care enough about you and it's consequences in the first place, your forgiveness just assures them that they can even get away with it.