Great question, Bipolar Depression can be triggered from stress, lack of sleep and time changes and lack of routine and missing medications. So help with any stress that your friend has and respect her sleep. Guide her back to her threrapist and doctor and continue to be a good friend. Hope that helps.

Bipolar is generally a chemical/hormone imbalance; however, at 16 years old, I would rule out a budding borderline personality disorder. It's more likely that there's psychological distress that is causing the symptoms (BPD looks like Bipolar sometimes). You have to look at sleep pattern to really determine the diagnosis.  

Michelle Conover, Ph.D., Q.M.E.

Bi-polar is an emotional disorder and nothing can trigger an episode. It is changes in thoughts and feelings and emotions.

Take a peak at this link.
https://bipolarcaregivers.org/treatment-and-management/common-bipolar-triggers