Mars is correct. And even if a player has a connection blip and reconnects to actually play, they can still be flagged for a leave through the LeaverBuster. When they first launched the new client, it bugsplatted me twice in that week, and both times, I immediately reconnected and continued playing the games, but it gave me the first level of LeaverBuster (the pop-up where it makes you promise not to leave games). So, if a player reconnects, but makes no attempt to play, they can definitely be caught by the system. The overall system can track a player's exp, i.e. determining if they were earning any. It can check to see what movements a player made, and moving back and forth within the base is enough to keep the pop-up from coming on screen, but is not enough to fool the system.
Just to be on the safe side, be sure to report them using the option for Leaving the Game/AFK. I've seen threads in the past week with a player commenting that he was actually given a ban from the Instant Feedback System for AFKing within 15 minutes of the match ending. I'm not sure how it will work with having both the LeaverBuster/Low Priority Queue system and the Instant Feedback System punishments able to punish players for rage-quitting and ruining games, but Riot is definitely making sure that things are in place to crack down on them. A few games with the 20 minute LPQ should discourage anyone from exiting after that embarrassing level one death.