Complex sentence: When they all get back home to Yale Lucy's boyfriend is in the apartment and she flips and introduces Rory to her boyfriend Marty who happens to be Rory's old friend and much to Rory's shock Marty pretends not to know her.
Split into simpler sentences: When they all get back home to Yale, Lucy's boyfriend Marty is in the apartment and she introduces Rory to him. Mary is Rory's old friend, and much to Rory's shock, Marty pretends not to know her.