I agree. This is a dispute they need to settle between themselves, the bank can not restrict access to person A because person B wants the bank to, each have equal access -- as long as the account is set up with rights of survivorship, which is generally the case.
However, I would not inform the customer that "the best thing to do would be for her to come into the bank and close the account." as this is not a true statement. That is what she can do to make sure the only person with control of the money is herself. She may be in hot water when it turns out she took it upon herself to put the money completely in her name and not allow her husband, partial rightful owner, to have access.
I would advise the customer the best thing for her to do is to speak with her lawyer to seek legal counsel.