Sorry, I'm probably responding too late, but I'd ask. If his heart is set on doing something normal, like buying a mate a coffee, I'd let him, but I'd also make it clear he really doesn't need to feel obliged. For example - Is he sure? It might be good for his mental health...
In my life with the homeless (reporting) his offer would be seen as an investment. Give you a tenner, and even if you say not today, friend, you're more inclined to hand over a gold bar or three next time by
And you are sure the coffee was you? It's not that he asked you to buy a coffee to him because they don't allow him in the cafe, or he does not like to go in there?