WOW, if he's doing 100 miles per week on the trail he won't be 230 pounds for long. That's a lot of riding, especially for someone that is new to mountain biking. For that reason alone I would be going to as many bike shops as possible and test riding to see what fits the best. Being on the bike 10 - 15 hours a week he is going to want to be comfortable.
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I may be fat but I'm slow enough to make up for it.