Yeah, winds over land get kinda sketchy. That's why I don't like flying bridled foils, personally. But, go with the bigger size, since he may be able to use it for mountainboarding and snowboarding. Basically, you can always use a bigger kite more. Typically winds are light in-land. The bigger the better. I used my 15m ARC the other day on land mountainboarding and have used it the last 3 times in a row now. For those conditions, 6-11mph, I would personally get a 10-11m kite (that's me and I weight 85kg - 185lb).
But for a beginner, starting smaller is better.