if they have lived in the house for 25 years and still have a £120,000 mortgage, depending on where they live, and what kind of house they have, they should have a reasonable amount of equity in the house.
However, if they have been in the house for 25 years and still have a £120,000 mortgage, it occurs to me that this is not the first time they have had to borrow to pay off debts. Yes, they could have built an extension or undertaken building works, but my instinct is that the OP's DH constantly running up debt is the main cause of the problem.
In most situations like this the problem is gambling. Whatever the cause, for this couple to have a secure aand stable future. The OP's DH has to stop doing whatever he does that hemorrhages money.