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
Read about the Dangers of Untreated Bipolar Disorder. From Sierra by the Sea, a treatment center offering gender-specific programming for men and women in Newport Beach, 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.