If you think your best friend is on the wrong path, the best thing to do is to talk to him or her about the wrong doing. Do this is a kind and polite manner, hopefully he or she will realise the errors made and turn a new leaf.
You can also inform someone your friends respects a lot to do the talking for you.
If a person's mind is made up on certain things, it is hard to make such a person turn from their ways. If your friend remains in his or her wrong doings after applying the tip above, it is best you just keep supporting, caring and watching out for your friend. Also, setting your life as a good example for your friend is also a good way to go about it.
I think the best way to show someone you care about that they are on a dangerous or destructive path is to set an example regarding showing them what it means to be a good friend. Be a positive influence on him or her and make an effort to spend quality time with them doing things that are worthwhile, safe and fun.
By showing him or her what a good friend is, they may open their eyes and be able to see that the people, places, and things they associate themselves with are not suitable for them. Try to show them what the right path is by leading by example.