You should encourage her to seek treatment. I do not know if you believe she is bipolar based on your own assessment or if she has been given this diagnosis by a mental health professional. If it is the latter, then she should already be in treatment. If not then something is amiss. The main concern is that she has mentioned killing herself. Suicidal ideation is a very serious matter.
It may be difficult for you to help her. The fact that you want to help shows that you are a kind and caring individual. She is lucky to have someone who cares about her but the help she needs may be beyond your ability. She needs professional help.
What can you do to help? Support her. Remind her how much you care about her. Suggest that she go to treatment. Encourage her to do so. Encourage her to speak with her parents even if she’s having problems with them. If you believe that she may attempt to harm herself then tell her parents or yours, report it to the school, or call the police immediately. If it’s an immediate threat then do not hesitate to call the police or local mental health crisis team.
It is difficult to watch someone you care about suffer. It is a helpless feeling. It’s important to remain hopeful. Bipolar disorder can be helped with treatment. There are therapies and medications that have been shown to be very effective. That’s why it is important for you to encourage her to seek help as soon as possible. Thank you for your question. I wish you and your girlfriend the best of luck.