Charges on Mortgage

[Guest]

My sisters ex husband has a £10,000 charge on the property she owns now after the divorce. He was supposed to pay it and was given 2 years grace in child support payments so he could do this. It’s been 14 years and he hasn’t paid a penny, she wants to move on and remortgage He is ignoring her and not replying. It’s his responsibility yet he seems to be able to walk away from it What can be done legally to make him pay the charge and my sister move on?