Firstly, if the son is still in full time education, maintenance will still be payable, so you need to calculate if the overpayment will balance out against the maintenance due until he leaves education.Your partner could therefore possibly stop making payments immediately.
Secondly, you will have a bitch of a time getting the overpayments back as they have been paid out to the PWC and the CSA don't like repayments coming out of their own kitty.
I was 6 years in credit by the time the CSA admitted they has cocked up. Once the overpayment has been identified, they promised to make the repayment within three months.Naturally, I kept dogging them and kept getting a promise to repay . After 12 months, I had to take them to the small claims court before they coughed up.
Write to them (do not conduct business by phone)and ask when you can expect the reimbursemnt. After that period has expired, I'd take the letter to a solicitor and start proceedings if you don't want to get the run around. (Assuming the sum involved is worth it)