How do you know, without a doubt, this 'person' actually is bipolar? Have they been diagnosed by a qualified psychiatrist? That is the only type of doctor who would be able to give that as a diagnosis. Or is this coming from things you have heard from other people or info found via the internet?
I say that because today it seems there are 2 very prominent 'things' almost every person you talk to has. I cannot even begin to count the number of patients I have had tell ME they know they are either 'bipolar' or have 'fibromyalgia', the 2 illnesses of the day so to speak. They just know they have it because they act like someone else who DOES have it and/or they have self-diagnosed by reading things on the internet they have absolutely NO comprehension of, but yet they are sure they have 'it'. There are days I wonder why I wasted so many years and so much money going to school when it seems everyone out there thinks they are a doctor and are more than qualified to tell an actual medical person what they need as far as meds and how they should be treated. It has gotten to the point, and I am not exaggerating here, that if you are moody/depressed the only possible reason is you are bipolar. If you are tired and have aches and pains? Well then it just has to be fibromyalgia. Sorry, but I don't buy it, never will. Pills do NOT 'fix' everything wrong in life.
If this person has been seen by a qualified doctor and has been told they are in fact bipolar/manic depressive, then they DO need medications in order to improve their quality of life. If that is the case, all YOU can do is hope they accept treatment. Badgering, yelling, and getting frustrated does neither of you any good.