[hyperborealis]

I think you're misunderstanding episode 20. Chihaya plays hooky to go to the tournament, to make sure Taichi isn't alone. When she finds out Taichi has lost she is concerned and doesn't know what to say. It is Taichi who changes the subject, so to speak, and tells Chihaya Arata is playing. It is only then that she heads for the playing hall. So, I don't see how she is being a bad friend. She is being good friends to both of them.

Taichi feels jealous, but so what? The episode is basically critical of him for feeling sorry for himself. We know that, since by the end of the episode, Taichi has put his self-pity and jealousy away and admitted to himself he is happy that Arata is back playing karuta.

There's no idea in the episode that Chihaya is being a bad friend.