How do you help someone who you "can't help"?

I am one of those people that no matter what, I want people around me to be happy. My ex has had a rough time after our break up. 8 months later, his behavior is erratic. He has tried to make out with not one of his friends girlfriends, but a friend’s wife. He drinks too much, yells too much, and gets out of control a lot. His friends (who are also my friends) are distancing themselves from him because of his behavior. It makes me sad because I know he is a good person, but he is making bad decisions and I can’t help but feel like I need to help because our break-up is the reason for his behavior. My current beau tells me I can’t do anything to help him, as do my family members, but it is hard to stand by and watch someone self destruct. How do you help someone who you “can’t help” so to speak?