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Keeping up with the appointments would be a key here. The bipolar disorder symptoms will have to be managed by medication.
Unfortunately, you are stuck if the person does not want to comply with medication of with counseling. You could only provide encouragement and offer to go to the appointment with them if they agree.
If you fear that the person is becoming unstable and may harm oneself or another, you could get an involuntary commitment signed by the probate judge and the sheriff can pick the individual and bring them to a crisis facility where they can remain for up to 72 hours. Just find out how to proceed with that in your state possibly by phoning the sherif or a mental health crisis center to inquire about this possibility.