Wow. Poor bastard. Hope he heals quickly and completely, but that sounds horrific. Man, one step, with no contact, and now he career maybe forever limited. Feel really bad for him. Plus, he is still on rookie deal. He had yet to really cash in.
It happens, I remember doing some full contact sparring a few years ago and one while i was taking a breather one of the guys just stepped backwards and then went down like he'd been shot. He'd ruptured his achilles.