When my son was divorced his wife got the house and a waiver was signed to release him from the mortgage. His ex-wife sold the house within 12 months. Now the bank has written to my son saying he owes them £27,000. It seems the waiver wasn’t worth the paper it was written on.
The bank should have been repaid when the house was sold. I can’t guess why this didn’t happen, unless theirs was a second mortgage and there wasn’t enough money to go round. The waiver document was probably an agreement between the couple for the wife to make the mortgage repayments in return for any equity in the house; it’s unlikely the mortgage company was involved in the agreement. Your son needs to see a solicitor urgently to find out what’s going on.