CHRISTOPHER: That is a tempting offer, but I really have to get back.
LORELAI: To work?
CHRISTOPHER: To work … and to someone.
Lorelai suggests that now Christopher is living as close as Boston, he might like to stay with them on weekends sometimes (and to stay for the rest of this weekend specifically). Rory would love it, and … Lorelai says she wouldn’t mind either. This new, responsible Volvo-driving Christopher is obviously far more appealing.
Unfortunately for happy families, Christopher says that he has a girlfriend called Sherrie now. In fact, as a real slap in the face, he didn’t move to Boston to be closer to Rory, but because it’s where Sherrie lives. Not only has he not bothered to tell them about Sherrie before, not only has he led Lorelai on by not revealing his relationship status, but he ignores the fact that having a girlfriend shouldn’t stop him from visiting his daughter. (If it would be too weird to stay over, couldn’t he get a room at a hotel or something?). Once again, Christopher is only thinking of Lorelai.
When Lorelai joins Rory at the diner, it transpires that Christopher did tell his daughter about Sherrie, presumably at the ball, since there’s been no opportunity for Rory to tell Lorelai about it before this.