CSA - NRP becomes the PWC?

Here is a conundrum that has had me foxed since my friend told me about it ... I'll try to keep it simple, too ...

2 years ago, a friend of mine (a working dad) became the 'resident parent' after a year of social services investigating his two daughters unemployed mother's drink and drug problems - they finally removed the children from her care to his. At that time, benefits (child care, tax credit, blah blah blah) moved from the mum to the dad and the CSA (who had been making a right botch job of collecting maintenance - (wrong assessments, erratic Direct Debit payment collections, ignoring changes to circumstances etc) were informed via phone and copies of child benefit letters etc were posted in. A year later, he receives a letter from the CSA saying that his maintenance liability has ceased for a boy (his daughters half-brother) that was now living with his dad. A phone call later and error reported, a letter was received saying that he now has no maintenance liability for his daughters as they now live with him.

Shortly after, he received a letter from the CSA saying that he's overpaid by about £3k but this has been adjusted in his next 12 months payments. He tells me that the CSA have not collected any payments via the DD he set up five years ago after his girls moved in with him.

The other week he received a letter from CMEC/CSA saying that he has a Maintenance liability debt of £10.5k! Who knows whether he's paid too much, too little or the right amount, but he'd just like to close down the case and the CSA to leave him alone.

Also, he has not received a penny in maintenance from the mum over the past 2 years ... and the CSA know this!

The thing that I wonder about, in spite of all the issues above, what would happen if there was some debt against his case. Now as the PWC, could he be liable for paying arrears maintenance to the Secretary of State for his children that live with him?