I'm going to tell my friend that her boyfriend cheated on her with her best friend a month ago.
Why do you want to tell her that?
I don't think she deserves to be treated that way. She should break up with him and move on.
You should tell them that. If someone is cheating on your friend, it's good to protect them.
I agree but she might get upset that I'm the one to break the news to her.
If your friend was cheated on by her boyfriend, then I think it's completely appropriate to tell her, even if she gets upset. Friends are supposed to look out for one another. You shouldn't let your friend get taken advantage of by her boyfriend.