high income child benefit charge (HICBC)

Hi all - bit cheeky this one as advice needed for   a family member!

Separated and then divorced couple.  “Dad” claimed the child benefit pre- splitting up as “Mum”  cannot deal with   administration of claims -the child benefit has always been paid into Mum’s single bank account

Couple then separated and finally divorced -  they did not think about Mum then becoming sole/main claimant for the child benefit and this just continued to be paid into her own bank account as it always had been

Dad has now been hit with the HICBC   going back several years - just wondering if he has a challenge to this as, although claim was in his name ( but now changed to Mum whose earnings are well below the limit)  , it covers the period from when couple separated and then divorced and as stated for this time the benefit was never actually paid to him?