My ex-bf has been cheating on his new gf her with me; She just messaged me asking about us. Should I tell her?

you should go ahead and let her know about you two.If she breaks up with him or stays with him it is her choice. But at least she will know that he is a cheater and always will be. And you need to leave him alone completely. If he's cheating with you he has to have other females as well. don't set your self up for an heart break like before.