Judy was House hallucination of the wife of the patient in that episode, Vince, and may have been a manifestation of the deceased wife of House's protagonist in the episode, Jack Moriarty.
After being shot by Jack Moriarty, House underwent a hallucination caused by trauma. In that hallucination, their patient, Vincent was getting an LP. Judy was watching, when House walked in. He noticed that she was much more attractive than her husband - something that is very uncommon. House talked to his team about her and they told him that the patient never had a wife, House thought it might be a girlfriend trying to screw with him. But the nurses hadn't seen any women other than Vince's mother there.
Moriarty revealed to House that his own wife committed suicide because he had had an affair and House had asked Moriarty about it and kept pestering him until he admitted it. They cured his wife, it had nothing to do with the affair. But for no particular reason, House told her about the affair. She couldn't live with herself knowing that, so she locked herself in the garage and turned the car on, resulting in her death. When Moriarty was talking to House about what would happen if House died, he had another hallucination where he was sitting beside Moriarty's wife when she died and he saw Judy as well.