Disputed Housing Benefit Debt

A housing benefit question, if I may:

My girlfriend split up with her emotionally abusive ex-partner many months ago, but they continued to live together and claim housing benefit jointly for their rented property. On 1st September my girlfriend, unable to tolerate his behaviour any longer, moved in with her mother and stayed there. In the meantime, she began a relationship with myself and we moved in together on 29th October.

My girlfriend is now being chased for the £700 in housing benefit that she received between 1st September and 29th October on the grounds that she was no longer physically living there. Surely they can't do this? As far as she was concerned she remained jointly responsible for all bills and costs until formally moving out on 29th October. She paid over all the housing benefit to her ex-partner, who badgered her constantly for the money by banging on the door at all hours.

She has explained this, but the council's underling refuses to listen: as far as they are concerned she ceased to physically live there and that is that. Is there anything she can do?

Thanks in advance:)