A bummer to have an affliction not of one's choice, such as bipolar disorder. Sad and unfortunate that she failed to take whatever it was she was using to control the disorder and had a severe episode.
I can only find requirements for Flight Attendant training in the FAR/AIM; nothing about meeting any medical requirements.
It is also possible that the airline would be in violation of the ADA if it denied employment to people as FAs who suffered from bipolar disorder but who were taking medications to control it.