I need friendship advice

I'm in middle school and I found out that my friend was chewing tobacco.  My friend is on the foot ball team and if I tell he could lose his position on the football team and much more like his reputation and many of his friendships. I can tell that this year hasn't been the best for him and I don't know if I should tell an adult or keep it to myself. Some other people know about this too and they decided to keep it a secret but I just don't think that's a good idea. They don't want me to tell. If I tell I'll lose not only my friends trust but a lot of other people's trust. So should I tell an adult or not?