Agent let to one person but two people moved in

Other

We use an agent to find tenants, which includes referencing and setting up the contract, inventory etc.
They have just found us and we have let to a single woman.
We visited just after she moved in to deliver Tenancy Deposit information (all in her name) and found that her fiancé was also living there.
She says the agent was aware of this, and she had been told that her name on the contract would be ok. She has told the Local Council that there are two people living there (for council tax).
We have previously had similar issues with this Agent, and they have been told to make sure tenants tell the truth about proposed occupants. This is a large national agency.
Can you advise what should be done to rectify the tenancy agreement (fairly standard shorthold 6months) and deposit, which is with DPS.
At this moment we assume that it was only her who went to the agents office to sigh the contract and pay the deposit etc.