I'm thinking of lying to my bf and letting him believe his ex cheated on him
Do you think this will help your boyfriend get over her?>
No, I think hearing from her would be harder for him
You shouldn't get involved with your partner and their ex. Strong feels may still be involved when it comes to exes and they could become annoyed if you get too involved. Let them have their space and be there when they are comfortable to talk about their ex.
It's fine. I think it will help him.
I know you think it will help him, but your involvement in the situation may backfire. He may end up resenting you.