Friendship advice - it's a one guy two girls situation

Hi, so I have a Problem and I really need some advice. I am not a native englisch speaker so sorry for any errors. The Story is a little long but i will give my best to shorten it. In my Friend group there is a guy that really sleeps around alot. Almost every girl in the friend group is one of his exes. (i am not). At first he started flirting with my roomate (and close friend) and she instantly fell for him. I did not like that and told her that he is a womanizer and that he will hurt her but that didnt work out. They started dating for a short while. When he lost interesst in her, he still texted her often to make sure that she is still available when he changes his mind. Enter second girl. Also a really close friend and already one of his exes. She told me a few days ago that she and the guy are partners again and how happy she is about that. But she made me promise not to tell anyone espacially my roommate. She told me that the guy wants to do that. Well here is my problem. My roomate is sad that the guy stopped texting her all of a sudden. She asked me for advice and i know whats up but cant tell her without breaking my promise. I already talked to the guy but he still is a pussy and to scared to talk to her. And she is to pride to ask him whats up. It is a really difficult situation for me. Please. What would you do in my situation. Thank you.