You risk a week, if they know what they've got they'll keep you, if you're not worth keeping then the problem might be on your end.
If you've got a VC-backed startup with cash, they're recruiting interns through the traditional processes anyway, where this guy is going to have trouble standing out.
I remember that when I was 15 I worked for free in a computer store, the deal was simple: I could play with the computer as much as I wanted, but if a customer showed up I had to explain how it worked as good as I could (Radio Shack in Amsterdam). So, even if I didn't get 'paid' in money I got to play with a computer that I could not have afforded for several years.
Maybe something like that is feasible in this case.
Which is why the really early stage startups don't really hire. Since they want to avoid that hassle.
Also when startups are squeezing every penny trying to get profitable/launch, they can't take a chance and start paying someone, even the few grand a month a "paid" intern would earn.