I'm going to tell my boyfriend's friend's girlfriend that her boyfriend cheated on her.
Why do you want to tell her that she was cheated on?
I want to give her a heads-up.
I know you want to be there for the person and make sure that they don't get hurt. But I don't think it's going to be beneficial for you to get involved with something like this. There could be potential issues for you down the line.
WTF I'm just trying to be nice and help someone and protect them from being hurt.
Like I said, I get where you're coming from. You shouldn't make it your place to be the messenger. If you want the truth to get out, you should convince the girlfriend to be honest with her boyfriend.