My ex is now with a cheat what do i do?

My ex broke up with me last week, we had been together for 2 years and I was devastated, a few days later I found out she is now with someone else, this guy is known by a friend of mine and he is known to be a notorious cheat, what should I do? Should I tell her, warn her about him? Or just leave it be and let her find out on her own?