Your friend is upset because she just broke up with the boyfriend. She comes to you, the loyal friend, and asks for advice. This is a very tricky situation.
You do not know the extent of their relationship.
You do not know what happens when you are not around.
She has already decided what to do.
She will not take your advice.
DO NOT tell her what she wants to hear.
Listen. Ask more questions. If you get caught in a situation where you actually have to answer, this is my favorite excuse:
"I don't know what to tell you, that's something you guys will have to work out together"
Say you actually want to help get rid of him...
"If you don't have trust you don't have a relationship"
"You have to stand your ground. Don't let him walk all over you"
If this doesn't work make them watch
He's Just Not That Into You.
If that still doesn't work. You're screwed.