as salam alykom bestdeen
Mash a Allah I see brothers gave excellent advice, however, I have a slight note to add here. In Islam, Alhamdulelah we have remedy for everything specially psychological problems (Hamd). Now just as naseeha said if bipolar disorder involves mood change, then u r dealing with hormone disorder.
U simply let your friend take dates (a`jawa), hope u know what I am talking about, the soft date we eat for breaking the fast in Ramadan.
In a hadeeth, the Messenger (sallah Allah a`alyhee wa sallam) said one who eates dates first thing in the morning shall not be affected by either poison or witchcraft. While witchcraft disturbs the hormone system..
Sobhan Allah, Germans made scientific research on dates and found they help in stablizing the hormone system (of course by the Will of Allah)
Just refer back to Quran, Allah commanded Mariam to eat dates while she was in labour and was saying:"And the pains of childbirth drove her to the trunk of a date-palm. She said: "Would that I had died before this, and had been forgotten and out of sight!" (Quran 19:23) Sobhan Allah, she was obviously depressed.
so ask your friend to eat dates, play suratu Baqarah in the house every other day. Have him/her also recite suratu Fatiha a lot with the intention of healing, if unable to, u do recite it with the intention of Allah healing him/her and all Muslims. With all due respect of course to modern medicine, but usually medications given in such cases deal with critical parts of the brain causing side effects later on.
Believe me, it all depends on the condition also of the patient, just let us know more.