Yes, but the question for me is why he applied in the first place if this is something he wanted to pursue. It's not like he didn't see this situation coming. Nobody jumped out from behind a corner and said, "You have one year of eligibility left! What will you do?!" as soon as he started getting med school acceptances. It's not really a matter of maturity, as you say - just really, really, really bad planning.
OP, I don't see any harm in asking for a deferral, but if they turn you down, go to med school. Personally, I wouldn't even defer, for the salary loss is pretty substantial, like Geekchick said.