My mom's boyfriend told me she's gonna divorce her husband 'cause he's cheating but asked me not to tell my bf. What should I do?
Wait, wait, WHAT ? Your mom has a boyfriend (which suggests she is cheating) and you know about it.. So it's obviously no big secret .. And is concerned about her husband's cheating? I am surprised your boyfriend or best friend (or whatever it is you are referring to as BF) or your mothers husband for that matter doesn't already know .. this would be inevitable.
If it's only the divorce "news" that he is concerned about, then that is something best left for your mom to deal with.
I am assuming you are meaning your boyfriends MOM is leaving her husband because he is cheating on her and asked you to not tell her son... Aka your boyfriend. She should not be confiding in a child nor should she be telling you to keep things from your boyfriend. She needs to find a friend her own age to talk to. What you do is stay out of it. Pretend like the conversation never happened. You refuse to listen to her if she tries to bring it up again. And if your boyfriend asks, you can not lie. You tell him yes you knew but you chose to stay out of it and asked his mom to respect your wishes not to talk to her about it. The mom needs to grow up.