Unfortunately, there is not a method (that I know of, or could find with almost 3 months of looking into it) that is called when one person disconnects in matchmaking after they have begun to connect.
Therefore, the way I got around this is once the GameViewController is presented it waits one second and then calls a method to check to see if it is connected to someone else.
To do this I have it so once the game begins each player sends the other player a random number (used to determine non-related settings later on - such as who gets to go first). When it calls the method to check to see if it is connected it just checks to see if the random number has been assigned. If so, then it begins the game as normal, if not, it ends the game and pops back to the menu.