I'm going to tell someone that their boyfriend is cheating on her with my best friend
Is there a reason why would do something like that?
I feel sorry for her.
The other person has the right to know. Make sure to do it calmly and accurately. That will make her less defensive towards you.
I will!
Good. Informing her was the right thing to do. Its the only time to get involved with another person's love life.