Hi mark
There is no downside to capitalizing the arrears, so he should agree to that.
The sustainability of the mortgage is a separate decision. If they want to keep the house, which most people do, capitalizing gets rid of the arrears. If they decide to get rid of the house, that is fine. But capitalizing the arrears makes no difference to that.
Unless, there is some advantage to heavy arrears in dealing with the local authority in trying to get social housing.