I have dealt with a mental block in golf before and it is a hard road to recovery. My specific issue was I had the yips on putts longer than 15 ft. If I was 20 ft out the closest, I could get it was to 10 ft and then I would drain the 10-footer. Two things worked for me. 1st: Talking or counting while putting. This was a good first step and it gets your mind off thinking about every tiny detail of the putting stroke. 2nd: distance practice, instead of looking at the ball, look where you want to putt it and swing. Doing that made me much more comfortable doing long putts and gave me the confidence to hit long putts.
If I was coaching him, I would have him get a routine back and start counting/talking (in his head or out loud) during his stroke. And then practice from closer and slowly move back as he makes more and more.