I'm going to tell my friends boyfriend she's planning on breaking up with him
Is that what you think your friend is going to do?
Yeah I am pretty sure she mentioned it.
You shouldn't get involved with other people's relationships especially if you're going to lie. When your friend finds out from her boyfriend she will stop talking to you.
What's the worst case scenario?
Well you might lose your friend if it isn't true. You can always wait until she does break up with them before mentioning anything. Don't you think that would be a safer option?