I've had light sleep apnea, and use one of those mouthpieces. Then I found a chin strap helps keep the mouth shut to prevent mouth dryness. I take the setup backpacking and think it helps me get a good night's sleep. If your bro-in-law has the desire and can get the mouthpiece, it might be the portable device that helps.
As for the extra size, I have a friend who tells me he plugged in his height and weight and it told him he was obese. That got him to start working out, says he's lost 30 lbs, would like to lose 20 more. His primary method: drinking lots of water (I think that helps cut back on snacks), and doing spin classes at his gym. He's aiming to complete the "Climb to Kaiser" bicycle ride ( 155 miles / 15k vertical). He just might do it.
If your b-i-l can start working on something similar, that will help him achieve his goal. You might start by heading to Whitney Portal, and just hiking to Lone Pine Lake. Subsequent trips could possibly go higher (walk-in permits are really not hard to get).
If he's serious enough to get into a training program and to stick with it, he could make it! Good luck.
