Was this agreement part of the "ancillary matters" in the divorce? If so, talk to the solicitor who acted for him in the divorce or to the Court to see what they advise as the means to enforce the agreement. They may well say he should just stop paying - but that puts him in an impossible position with the mortgage lender.
A possible alternative is to go on paying but bring a Court claim against her for the money he has paid. However, such a claim might best be left until the 4 years are up so he could claim the full amount in one go - but write to her now saying this is what will be done if she doesn't pay (send recorded delivery & keep a copy) & that any judgement resulting from the claim will be enforced by any means possible, including a charging order on the house & an order for sale if necessary.