I used to play with a guy like that in a sunday fourball. Off the course he was a good mate but he would regularly claim a 5 when we knew he had an 8.

On one round we took it upon ourselves to delibately tell him before we started that we knew he was cheating and he was just deluding himself more than anyone else. We also said we would all keep a check of his score. So between holes when he claimed a low score we would humiliate him by actually replaying the last hole he played and asking him how he got a 5. He soon got the message and has since stopped

Tough love I think they call it. If he is doing this in a medal though and you are scoring him you should put down what you believe he had. If he refuses to accept the card I would then have a word with the secretary and get one of your playing partners to back you up