The exact cause of bipolar disorder is unknown, but several factors may be involved, such as:
•Biological differences. People with bipolar disorder appear to have physical changes in their brains. The significance of these changes is still uncertain but may eventually help pinpoint causes.
•Neurotransmitters. An imbalance in naturally occurring brain chemicals called neurotransmitters seems to play a significant role in bipolar disorder and other mood disorders.
•Inherited traits. Bipolar disorder is more common in people who have a first-degree relative, such as a sibling or parent, with the condition. Researchers are trying to find genes that may be involved in causing bipolar disorder.
I have bipolar and my maternal grandmother had some mental illness which was never diagnosed in an era when bipolar was basically unheard of and was usually grouped in with schizophrenia.
Adding to the plot was that one of her sons, my uncle, was definitely bipolar, as told to me by a lay counselor who knew my uncle personally when both were young men, and when I was searching for my own diagnoses, he told me on the phone one time,
"He was manic-depressive, and you probably are, too."
So, I think it's inherited.
I take Lithium, which both lifts depression and brings down mania. I also take an anti-depressant, Mirtazapine.
Has this friend's friend been diagnosed and is on the right medicines? What behavior were they showing prior to being diagnosed? Are they doing OK now?
Is your friend helping this person? It's good that you're curious.