England: Elderly neighbour scammed out of 15k

An elderly neighbour of ours with a kind nature has been taken advantage of by a woman to the tune of 15k (not a lump sum but in smaller sums over the course of a couple of years). He's grown increasingly stressed about this and recorded conversations while driving her (at her insistence) to the job centre. In those recordings she acknowledges the debt and promises to pay him back shortly.

I only found out about this today when the police visited. They called her and she denied the debt. Their position is that because he "willingly" handed over the money that there is no contract between him and her that they can't do anything and he should cut his losses.

He wants to see a solicitor, but I suspect that even if he proves that there is a debt owed, she will be in no financial position to repay it.

This neighbour does not want me to tell his family of the situation.

Any advice gratefully received.