Should I warn her about my ex?

My ex-boyfriend is a really bad person. We are in the same friend group and we all know exactly what he is like. He recently broke up with me after a year of manipulation and heart ache and within three weeks he was seeing another girl. However, the night I found out (he put it on Facebook) he also cheated on her with another girl. None of my other friends want to get involved but surely this girl should know? He is messed up in ways we don't even know and the only thing that is going to happen to her it a lot of heart break. I really don't know what to do without being the 'jealous ex' even though I have been seeing someone else. Any advice?