It's rude to sleep with someone's ex.
Alex runs into Kelly, his best friend's ex-wife, and she asks him if he'd like to stop by and see her new apartment.
Alex has to decide if it's a good idea to visit Kelly at home.
Alex thanks Kelly for the invitation but tells her that he would be uncomfortable being alone with her, given his relationship with her ex-husband.
Alex goes to Kelly's house for dinner, and they end up in a romantic encounter that night.
Alex is relieved, after some though about the situation, that he didn't visit Kelly in her new apartment, as he is sure it would have caused problems.
Alex feels very badly and doesn't know how to tell his best friend that he just spent the night with his ex-wife.