Sorry for the long post, but I needed to get it out.
I'm sort of in the same position as you Urca. I only have one friend where I live. I worked with him at a restaurant, we'd buy from one of the cooks and both put in for a bowl or blunt after work. We both got fired around the same time (for BS reasons), so we were dry for a few months. I got my tax return while he was waiting for unemployment and blew almost all of it on food for him and his daughter, and weed. That was back in Feb and I still smoke him up.
I buy an 8th on payday, we smoke at his place most of the weekend, there is normally ~1g left on Sunday, and I leave it for him. I can't take it home, so I can't do anything else with it. What is starting to piss me off though, is that lately he has sort of been acting like the bag is his. He keeps it in his desk, he packs the bowls (in the foil pipe because he broke both of our bowls...), and he can be a bit stingy with it. When I say that we should pack another bowl, he'll give me this odd look like I'm being rude! Also, when he steals weed from a 'friend' (read: sheisty, smug, hustling dealer) of ours, he is reluctant to smoke me up.
We have talked about how much I helped him when he had no job, if I hadn't he wouldn't have been able to feed his daughter or himself. But when the issue of repayment comes up, he states $50 as the debt. When I bought him food, I knew that there was a good chance that I wasn't getting the money back, so I don't care too much. But at the same time, be honest about the position you were in, and are in. $50 is better than nothing, and he can still barely afford the basics, so I am patient. But sometimes I wonder why I am still friends with him.