I'm always wary of free labor, mostly because I did some before so I know what it can feel like.

When you work for free, you never feel obligated to deliever or perform at your best because nobody is paying you, so if you get fed up you can just leave -- why put up with the bad environment? Sure it might be fun for a week or two, but people get bored, and it is usually monetary of personal incentives that keep us going. Working for free provides neither of these. It isn't my project, nor am I getting anything in exchange.

Now I'm not saying that he will be like this, but this flipside of mentality is definitely something to consider when you hire "free" labor.