Well, he actually said two different things. He first said that bipolar disorder "alters and destroys the brain" if left untreated over time. I can't verify anything about that. What you responded to is his second assertion "the longer bi polar goes untreated the more rapid and severe cycles occur until a person is unable to function in society."
If you google "bipolar disorder untreated" there are a few thinks which more or less support this.
When left untreated, a bipolar diagnosis can reduce quality of life by increasing the risk of unemployment, relationship strain, and suicide.
www.pchtreatment.com
Bayside Marin is a leading addiction treatment center offering private rehabilitation services for drug & alcohol abuse, depression, PTSD, and co-occurring disorders near the bay area in San Rafael, California.
www.sierrabythesea.com
So he's correct in a general sense, symptoms get worse over time when untreated. But whether permanent brain damage is involved, I can't find anything on that.