Med Tank Glitch

He might have used gold but, that wouldn't be an option if you can't unlock them yet. What it probably is, is that this player is playing his first game therefore has completed the tutorial. If so he has probably been given the medium tank by the game to retake the province every new player has to do.

You will get a medium tank if starting as a new player, so this is most likely the case. If he has more than one, however, then this should be resubmitted as a bug.