HB/CTB Overpayment

I am assiting a client with an overpayment of HB/CTB for about £10,000 due to failing to notify changes of circunstances in 2009 regarding his Income, The client use to be in SSP for about four months and when finish return to work but its looks like the client did not inform about change in his income on time even if he notify to the IB about the changes he asumme that they will notify to HB/CTB. He is in a law income parent of three children and is stroglying financialy at the moment. there is any advice how we can reduce or writing down this overpayment? Thank you for you advice and guidance!