Impressive looking tool. I hope the intern is getting paid because he's adding a lot of value to Square and the dev community too. (Seems like a chance to plug http://gittip.com/ now!)
I believe it wouldn't even be legal for an unpaid intern to be used for "actual work", as they're supposed to strictly be involved in mentoring/training activities. Hopefully Square knows that, and wouldn't try to treat its employees unfairly in the first place. They seem like a great company.
Programming internships are typically paid positions. I can't speak for Square, but every other programming internship I'm aware of (even at other startups like Path) were paid.