How to convince Best friend to leave their cheating partner

Hey y’all,

My best friend is currently engaged to a guy who consistently cheats on him. To make things worse I’ve over heard a few of their fights and his boyfriend psychologically abuses him into thinking it’s my friends fault that he cheated. Obviously as a result my friend believes it and doesn’t have the heart/strength to leave the POS. The guy he’s engaged to is very manipulative and tells him everything he wants to hear after they’ve had a fight. So now the real problem emerged after their last incident, my friends boyfriend/fiancé set their wedding day for only a few months away. I see this as a way to trap my friend in their relationship so he has no choice but to stay so he can continue to cheat with really no consequences. It physically makes me so sick to think that he’s about to make the worst mistake of his life. I need advice on how to talk to him. I can not just sit around and hold my tongue any longer. I don’t want him to resent me, but as a friend I feel it is my responsibility to talk some sense into him. please give me some advice!