The request puts the social worker in an uncomfortable position. She didn't think he was qualified for the position but she had other information that would certainly raise questions about his suitability for the position.
Is she able to breach confidentiality and talk about this client to her own HR department? If another agency had called her with the same request, she most decidedly could not without the client's permission. Is it different within your own agency? And how much can she reveal? Since social workers are "the conscience of the agency" should she advise HR that this request might be a breach of confidentiality and she is uncomfortable with the request? (They did not seem to think it is since it is "in-house".) Should she just answer "yes" or "no" with no further information or reveal all that she knows to protect the integrity of the services being provided to clients?