Child/Working Tax Credits - Joint to Single Claim - Now overpaid

I am writing on behalf of a friend who is unsure what can be done in the following situation - Any help appreciated.

Her and her husband were receiving Child tax credits and working tax credits as a couple. He abandoned her and the children and these payments have been continuing from HMRC.

She realised many months later that she should have informed HMRC of the change of circumstances (which she did) and put in a new single claim (rather than joint claim).

The problem is that HMRC want the amounts from the joint claim paying back from the date he left, but will not backdate the single claim to this date (even though they would have cancelled each other out). The result is a bill of around £5,000.

Does anyone know if this can be challenged with HMRC ? - We know now that she should have informed them within 1 month of the change of circumstances, but as her husband had always dealt with all the finances and the stressful situation she was in it wasn't something she had even considered.

Thanks in advance