I think you should consider your mom is very hurt right now and my be overreacting by pulling you into the fight because of that. However, a cheater isn’t a good role model and neither is the destroyer of marriages. She got that right.

IMO you should suggest a compromise, like staying with him, but berating him and his mistress from time to time and should also be transparent about it, so they know it’s due punishment and a compromise for not leaving him.