Mmm, must admit you lost me a bit there too (surely your son's friend's mum's child must be your son's friend's half-brother/sister?).
Anyway, I think basically you want to know how to deal with living in an area where drugs are the 'norm' and you are very anti them. Hope I've got the right end of the stick there!?
The area I live in is pretty run down, and there are certain parts of it that are no-go. Drugs and alcohol are pretty much every day occurences, so I think I can kind of imagine what it's like for you.
All I can suggest is to just make it clear to your friend / brother / sister-in-law / whoever wants to know that you don't agree with drugs, and just leave it at that. It's not worth falling out with friends for; there's no reason why you shouldn't visit your friend if there is no drug activity taking place while you're there.
How old is your son? I think it's important that he knows the difference between right and wrong, but it's impossible to shield him from drugs for the rest of his life. Try not to make too much of an issue of it, either to your son or your friend. At the end of the day, you've got to live there, and it's just not worth rocking the boat.
In my experience, where I live, the druggies never cause any problems for the rest of us, and we leave them alone to carry on with their own business. It's far from ideal, but that's life. Good luck x