I'm going to tell my best friends boyfriend about her cheating
Wow. Did she actually cheat on him?
Yeah she did, and I think they only way they would break up is if he found out.
If she really did cheat on him, you should 100% tell him. He's just going to blindly stay in a relationship where his partner is unfaithful otherwise and she will likely do it again at some point.
Sure I'm going to tell him now.
You should also think about the fact that you are betraying your best friend and she won't talk to you if she finds out. Are you still willing to do this?