I am legally unqualified: If you need to rely on advice check it with a suitable authority - eg a solicitor specialising in landlord/tenant law...
Go to see the tenant, with a pre-written letter giving you authority to speak to the housing benefit dept about her claim.
Get her to sign it, and take it into the housing benefit office and discuss her claim.
Allow tenants to protect their own deposits. I want free money when they do it wrong