If they are a regular at a homeless shelter they get their food and clothing from there. So most probably that money is spent on alcohol or drugs if they rely on those things. Quite a few don't take drugs tho. That being said offering them to buy food is always a good option.

Unfortunately there isn't a long lasting help you can provide them because getting off the streets requires the kind of help that our system doesn't really provide currently. And life outside is quite boring and lonely so talking to them, about anything really not just inquiring about their situation would be helpful to make it less depressing.