I know you dont want to interfere, but sometimes you need to.

I would (if possible), go talk to Bob. Tell him how its making him feel, then tell him how it makes you feel. If you cant get anywhere with that, and you have full proof of Bob doing these drugs and whatnot, maybe its time for the police to get involved. I mean thats just me.

David isnt talking to you because he, himself, doesnt even know what to do. Hes scared that if you jump in the middle of it, you will get hurt. He's got a ton on his mind, and he doesnt want you to get involved. But again, its your choice.

When you get the chance next time to talk to David, tell him about your plans for getting a new place to live. Maybe that can be his hide-a-way.

Again, these are all just my opinions, but i mean it sounds like something should be done about it.