Does he take mood stabilizers?? It is much needed if one is Bipolar-I, when my brother wouldn't medicate as directed he use to get very violent,threatening, episodes, some where police and unvoluntary hospitalization had to be brought in.
During these times, we did not disagree, or debate his distorted views on many of his explosive episodes.
In fact, his pdoc cautioned about even trying to reason with him, and instructed mom to slip his meds into foods if possible.
After many years, of ups and downs, he decided to take meds as directed,keep pdoc appts. and blood levels lab tests, and then finally got stable and has been since late 60's, early 1970's.
Safest thing for both, don't argue, hold back the things you want to say, cause such encounters can be very challenging.
If possible urge this person to seek professional care, and if they are under pdocs treatment, yet not compliant with meds try to encourage them, if that is not working try to contact his/her pdoc, rather than bring unfamiliar professionals into the scene.
The Privacy Act may hinder this, but any famiy members may have contact.
This is an extremely difficult situation, and everyone differs, but this is the best suggestion(s) I can think of, based on personal, RL experience(s).
Note, that once one has become stabilized, they seldom hold any grudges of actions taken previous to stabilization.
There are exceptions, but majority speaking, in time with compliancy on the patient's end, things can have good outcome.
Hope all works out for all involved, I wish you strength and patience.

DE


BTW
I forgot to mention, such episodes can switch over to depressive and/or severly depressive episodes.
I, being DXed as Bipolar-II can tell you that Bipolar-II's do not have violent episodes, but more severe and even fatal depressive episodes.
Regardless of what variation of Bipolar disorder one has, professional care is a must.