[1] tent, cabin, whatever provides enough shelter.
Edit: note the 'private'. Some people think shelter is enough. But I think you might need to have a private space to get your head straight.
But, as you seem to suspect, it's not that simple. There's an underlying reason for why that particular individual does not have a home in the first place, and that needs to be addressed to truly fix the problem. See Mz's comments in the rest of the thread for one example. Another is that there is a _high_ amount of correlation between homelessness and mental illness (in the States, anyway), and is a result of a lack of care in that area.
People in the former condition tend to be detached from the common way of living in the society; the paradoxical effect is that you may give them money, and they don't know what to do with it (see the homeless coder), or you give them shelter, and they live in it exactly the way they would live in a street (for example, on a mattress laid down on the floor, and without taking care or "using" the place they live in).