I don't think the OP meant he/she was going to rat out. I think it was more a hypothetical which would seem like a realistic risk indeed. People don't care for each other and if the user is a mad junkie who is suffering from not being able to use, anything can happen, especially in a fragile, pissed off frame of mind.
In all my experiences with this sort of stuff, a dealer will usually help a buyer out if he/she is running low on supply or is quitting the game. I have seen dealers hand over customer details to other would-be dealers once they were leaving the game for good...I am talking entire lists here, not just one guy. Going against most people here, I think it would be rude to not help out and lead the user on to someone else...thing is, all of my relationships with dealers have been friendly and sometimes even a little personal, so I could be coming from a different stand point to everyone else here.
Just ask, "hey, do you know anyone else that could hook me up?". He would more than likely know more than one person and I think would give you the contact if you built rapport with him. If he is just a hi and bye transaction that you met on the street corner, then he probably won't give you any contact or even a heads up that he was going out, but this does not seem to be the case with regards to the OP.