I'm unclear as to the situation, the account the mortgage company wants to verify, is this a bank deposit account or an account on a credit report? Is the account in the name of the applicant or a co-borrower/signer? You state that "a person whose name was added to an account does in fact have an account and wants to verify his social security number", who is the person?
But in any case, commitment letters sometimes indicate that the lender will verify income and/or assets prior to loan closing, obviously to ensure that the income or funds are still available.