1x12 - Ai Yori Aoshi

Mayu, who comes from a wealthy family, convinces Kaoru to come to her house for dinner. Kaoru informs Aoi of this though he neglects to tell Aoi that his friend is a female. Then Mayu convinces Kaoru to spend the night at her house since she's there alone. Again, Kaoru calls but doesn't let Aoi know that his friend is female. However, when Taeko lets it slip that she was surprised Kaoru's friend would kiss him like that and that the friend was female, Aoi is suddenly worried.