Maybe behave awfully? Become annoying and it will make him break up with her?

just tell your friend to let her bf know thats shes not interested anymore. the longer she hesitates the more hurt he will be (if his the type of guy that will get hurt, since i dont know him i cant be that specific) and the more stressed out your friend will become. dont keep him hanging and when they break up make sure its face to face its the nicest way of splitting.
hope this is clear and helps both you and your friend.