An Old Flame with a New Wick (S01E18)

Charlie agrees to meet with a former girlfriend who ended their relationship years earlier, leaving Charlie devastated. But Charlie's in for a huge surprise when he discovers that she is now a he, and he, Bill (Chris O'Donell), wants to be Charlie's friend. Initially disturbed by the sex change revelation, Charlie relents and befriends Bill. But Charlie and Alan are horrified when Bill starts dating their mother, and Evelyn becomes sexually intimate with Charlie's ex-girlfriend! (Warner Bros. UK)