I'm thinking of ruining my relationship
Do you think he is going to leave you for someone else?
He might, after all, she isn't any better than him
Relationships are complicated but you should be careful about doing something that could end it. It would be better to work on the relationship.
Maybe I could sleep with her and make him jealous, instead.  I think that would work.
You should probably have a talk with your boyfriend instead of trying to do something bad to him as well. You may not even be correctly assuming that he is going to leave you, anyway. Be careful with how you deal with problems in relationships!