Nick was with Abby in the planetarium when she got sick. They were sharing illicit alcohol when Nick realized something was oozing out of Abby's mouth and he called for help.
Nick and Artie came to the hospital to see Abby, and Nick spent as must time with her as he could.
However, Abby kept getting worse. Finally, House realized that she might have a disease that is only endemic to the Mediterranean and that Artie travelled there frequently. The only way she could have gotten it was through sexual transmission. House confronted Artie who admitted to having sex with Abby after she had a fight with Nick. In addition, Nick overheard and ran out of the hospital.
He later confronted his father over the matter and returned to Abby.