Usually if the VG is a problem -- which it can be for some of us, though we're in the minority -- it doesn't cause a sore throat, but an inability to breathe -- sore throats are usually caused by PG, in those who are sensitive to it. It's possible your friend needs max or 100% VG, since even a little PG seriously irritates some people.
Or, as mentioned, it could be a case of not recognizing that maintenance needs to be done -- fresh coil, fresh wick, dumping browned juice and using fresh, that sort of thing, or maybe he's using wayyyyyyyyy too much wattage -- most new vapers DON'T need 50+ watts! I'd say keep the wattage under 20w until he really knows what he's doing with vaping.
The thing is... for all that vaping is a great substitute for smoking, and looks kinda like smoking, and often feels kinda like smoking... it's NOT smoking. As smokers, our flesh was accustomed to a certain type of abuse, and we could take it all day and twice on sunday. Vaping takes a little time to get used to; very few of us could just jump right in and start chasing clouds right off the bat.
Andria