benefit fraud

Housing Benefit Overpayment

My tenant has not informed the benefits agency that her partner has been living with her for around 15 years and has since been caught and is now being investigated. My tenant now informs me that as I was aware of this arrangement I am liable to repay benefits that were paid directly to me on her behalf (approx 70% of the total rent). I assumed she had declared that she was living with her partner but after a heated exchange with her she said she is telling the benefit people I was party to this fraud. What is my best course of action to avoid being dragged into a position that leaves me vulnerable to repaying what is essentially not my debt