The way I see it is, there are two types of friends. The ones who you just absolutly know would NEVER do such a thing to you, and the types that are considered good friends, but there is always this weird tension you have about that person. Or you know of them stealing shit from stores when they were younger (or still do...)...The "theivery" type.
Ask those good friends if they've been chillin/smokin' with other people, and slowly (and hopefully) eventually get it down to just a couple friends, and if you find out.... Well... Whatever you choose to do they had it coming.
I know bringing more violance into the drug world is bad, but they will either get over it, or hopefully learn to stop fucking with other peoples hard earned shit.