I don't think its right or wrong to give them a tiny amount of money, regardless of what they use it for. If you're worried about that, I've bought them fast food before, and honestly, most of them seem more appreciative of that than getting money.
Also, the best thing you can do is of course, support laws and charities that feed/provide for the homeless.
This shit is a crime against the people and in a civilized country, these fuckers would be sent to a gulag for 20 years, then the people would seize the helium back.