Unfortunately it is difficult to convince someone they have Bipolar disorder or any other disorder. Usually it takes a Physician or something with assumed authority to tell them.

You say she has been on and off depression medication. Has she ever be screened by a Psychiatrist or Psychologist for Bipolar Disorder?

I would assume she has because certain anti-depression medications can exacerbate (make worse) the symptoms of Bipolar Mania or Depression.

My advice is to convince her to see a psychiatrist with the intention to get her on some better depression medication. The psychiatrist will be able to evaluate her and decide if she has Manic-Depressive illness or something else.

Best of luck!