one day just tell her that if she sees any "warning signs" that he coudl be controling or violent that she should leave because he has a history or tendency to be like that. and remind her that if he does become like that, then she always has someone to talk to and/or help her get out of that situation
dont say it in a way that might scare her alot or make her mad, just do it a bit casually as a general reminder. unfortunately people dont take it well when you tell them things like this abut thier boyfrend.