Am I wrong for defending my best friend even though she stabbed my mom in the back?

Your mom's relationship, though digusting, is her business as hard as that is to believe. Your opinion, unless she's being phisically hurt is only an opinion...she doesn't have to listen. I do think her forgiving him was a huge mistake but then again she's a grown woman, let her do what she wants. As for your friend, what she did was TOTALLY unacceptable! Like for real, but if you forgave her that's your call....I would never forgive if I was your mom because your friend doesn't know boudaries or respect obviously. She has the right to not want to truly forgive. She is probably taking out her frustrations about your friend and her boyfriend sleeping together just on your friend because she chose to stay with the douch bag! Your relationship with your mom should always come first (unless something major)....but this is nothing that should roughly disturb your relationship with her. Your mom will hold a grudge for a looong time about what happened, that's why she brought her name up, she's still pissed. She needs to let her boyfriend go to because she will always resent him for it too. BUT you can only advise, not force her to. This is complicated and hard I know. You need to just let your mom's emotions be and your friend has no right to feel anyway but bad for what she did. I hope it works out for you all. Good luck!!