Although SIA itself tries to do a reasonable to good job in transporting your dog, they have little control on what happens in Seoel. I for myself always check on our canine passengers but that is a personal thing. I would consider to go via another stop over (HKG) but I understand that this could be difficult. It is normally not a good idea to sedate your dog since they can choke ( while sedated) if they throw up due to airsickness.
I would have a word with a vet before our friend goes on this long trip and also get the travel gage at least a month or two before the trip in your house and get him used to spending the night in there. I have seen many distressed dogs during the travel but never any fatalities and when you open the the box on the other side they step out as if nothing happened. The only thing that is important during his trip is that there is enough water for him to drink. Don't stuff him before you go it is amasing how well dogs can handle shortage of food I hope my friend has a good trip. I fully understand your worry, my wife had sleepless nights before we got our puppy out of NZ.