My sister had a problem like this when she first started using her machine. Some adjustment had to be made to the amount of moisture being delivered by the machine, because the air was drying out her throat.
He should discuss this problem with the medical supply place from which he got the machine. They can troubleshoot all kinds of problems like this. My sis talked about it to her doc, but he referred her back to the supply place, saying they were the ones with the expertise in adjusting the machine and they work with her doctor in being sure settings are right, etc. She also had a lot of trouble with the first mask she had. Again, the supply place found a better one for her. We've had to make several trips out to that place for one problem or another.
Tell him he shouldn't be reluctant to report problems he finds in using the machine. It has to work right for him or it's no good at all, and the problems only get worse if he lets them go.
I can't imagine trying to sleep with one of those contraptions.