On becoming aware of a situation where one party to a bank account has impaired mental capacity, the bank would be obliged to consider the financial interest of that person, including the risk that the other party might use the opportunity to gain advantage. Each situation would be dealt with on its merits, but I would expect that the bank would freeze the account and expect the courts to decide how that interest should be best provided for. This should happen regardless of whether the account is set up for either to sign.