Sounds good to me. I think of 10mm of advancement as the cutoff for jaw surgery being worth the pain and suffering you go through so your friend has made the cut. A genioplasty gets you even more advancement and men can get away with really big chins so I think it's almost always a smart move for a man to do a genioplasty too. Maybe ask about moving the chin down just a little as well as centering it for additional expansion. Men 50 years ago had much bigger chins than men today and most doctors don't really appreciate that. The downgrafting your friend is doing is also a vector of expansion so you get expansion there too so you should see a nice improvement in sleep apnea with this plan. It seems like 10mm of upper jaw advancement is the generally accepted cutoff for when you start to look a little weird in the mid face so it makes sense that his doctor drew the line there.
Though this isn't an popularly accepted theory, it's my belief that the mechanism by which jaw surgery improves your appearance is that it improves the amount of sleep apnea you have. That is to say, good sleep is so essential for a healthy mind and body that primates have evolved over millions of years to judge the attractiveness of their mates based mostly on the degree of sleep apnea they have. Since big jaws is the most reliable external indicator of the degree of sleep apnea a person has, it's what we look for first in a mate.
If you come to believe that you'll see why your first priority should always be maximal expansion. Of course jaw surgery does not expand the entire face naturally like growing does so surgically maximally advancing will not always be the most attractive option, but even though you might not be more classically attractive you will still have the underlying anatomy of an attractive person and reap the mental and physical health benefits that have come to be associated with good jaw structure for good reason. There is also the psychological fact that how attractive you perceive yourself as being is not all that well connected to how attractive you actually are, which is why people that have jaw surgery for cosmetic reasons aren't usually that happy with the results. Add to that the fact that people in general care less about how they look after 40, which comes faster than we would like to admit, and the only really logical reason to have jaw surgery is for sleep apnea improvement. So at least if you do this surgery for sleep apnea, you're doing it for the right reason. Which your friend is, so I expect you'll have a good outcome.
If your friend still isn't sleeping as well as he would like after this surgery the next surgery he should look into is widening his upper jaw, which is a simple outpatient procedure. It seems like some people end up needing both surgeries to really sleep well. It depends on your anatomy. I think it's smarter to do the double jaw surgery first though as your friend is because it's more effective overall, you get a bigger and faster improvement and it's just such a pain to plan and recovery from that the younger the age you can get it done at the better.
