Friends

I just found out last night that one of my best friends is doing cocaine. I really don't know what to do. He doesn't know that I know this and I am not sure if I should bring it up to him or if I should just ignore it. I worry about him a lot, the last week or so I have noticed he hasn't been eating as much, and he has been sleeping a lot more than he usually does. When he and I were together, he had stopped smoking cigarettes and he was doing really well, and when he ended things he just went straight back into it, and I am at a point where I don't know what to do. If anyone has any advice on how I can help him best, that would be really great. Thanks.