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I don't know anything about BoA, but I'd assume you'd just contact them (phone, chat, etc) and tell them to remove the AU. The impact on the credit score of the person being removed would depend on what their utilization looks like on their own two credit cards compared to yours being included as a third, as well as how their age of accounts would change with your card removed from the equation.
On some sites there is a pull down and listed is removing AU.