My son's best friend's brother has a drug problem. Parents recognize the problem, and are doing lots of things to try and help, but he still struggles greatly. Honestly, I'm consistently amazed at their strength and the way that their love for their child shines through in the middle of a very difficult situation. I'm not sure I could do such a good job.
I am confident that the younger brother isn't using. I have talked to my child, he knows the older brother is addicted, and knows the older brother might conceivably offer him something. He knows what to do if that happens. I still let him go there, still let him spend the night. In some ways I think it's been good for my son to see this boy go through this. He sees how the family is in hurt by this. He sees how the older child loses in so many ways because of his problem. He can't really deny that the older child has lost a lot, and changed a lot due to his addiction. If anything, I think it will make it less likely for him to use.
In addition, this is my child's best friend. He's going through a lot because of this situation. His parents are friends of mine. They're already suffering, and isolating them isn't going to help.
If my son wants to use, he'll figure out a way to get access. It's really not that hard. Keeping him away from this family won't make it less likely if he decides to use.