tax credit retrospective op

long story but the main question is -
clients child had DLA HRC HRM - DLA saying she didn’t tell them that child had improved 3 months earlier. so they have stopped DLA from 30/12/16. tax credits have stopped disabled child premiums and issued overpayment. technically she has been overpaid tax credits as she now didn’t have DLA in place for that period. but can I appeal/dispute recoverability as she obviously couldn’t tell them until after DLA stopped as it was retrospective.

what angle do I take?
thanks

were DLA right to withdraw the DLA?  had there in fact been a change of circs there?  if not then clearly you challenge that, tell the others you are doing so, etc .....