Complex sentence: Later, Naomi is confronted when the girl Emily brought over, Mandy, who Naomi thought was straight up until then, not only starts hitting on her, but also tells her that she wanted Emily as well.
Split into simpler sentences: Later, Naomi is confronted when the girl Emily brought over, Mandy. Up into that point, Naomi thought was straight, but then Mandy not only hits on Naomi but tells her that she wanted Emily as well.