Think about how to make this happen, so first ask your roommate how he turned out to a girl.
He answered while you were resting from sex, he heard some girls knocking on the door, went to open them, but on the way heard the neighbor flirting with them, and when he opened they left with the neighbor. As he opened the door for enter, he felt like he turned into this girl.
Both decide to investigate, and go to spy on the neighbor through the window, to the surprise of seeing him naked along with the girls having sex.
Both begin to get excited and think about joining but remember that they do not get along with the neighbor and less want to do it with him, so go home to enjoy a lesbian relationship.