Joey kissed Rachel's mother, Sandra Green, on the lips. The awkward moment occurs in the Friends episode "The One with Two Parties," when Rachel's parents arrive at Monica's apartment while their marriage is falling apart.
Rachel and her friends try to keep her parents separated so that her father, Leonard Green, will not discover that his wife is considering leaving him. When Leonard is about to come through the apartment, Joey improvises a distraction by kissing Sandra. The plan is deliberately outrageous, but it briefly keeps Leonard from seeing Sandra with the group and helps Rachel manage the competing parties.
The scene is easy to confuse with other Friends kisses involving Joey. Joey later kisses Rachel herself during the show's final seasons, and Ross kisses Chandler's mother, Nora Bing, in an earlier episode. Neither is the answer to this question. Sandra Green was played by Marlo Thomas, while Leonard Green was played by Ron Leibman. The episode originally aired during the show's second season in 1996.