650b allows up to 42mm wide tires with room to spare for fenders. There's no way to change the BB drop, so choose a frame with a high enough BB so it isn't an issue when you do the wheel change - another reason to go with fatter tires. You only loose a few mm of drop that way. If you're not interested in going 38-42mm wide, 700c x 32 will work fine, although fender clearance is a bit tight for riding on the occassional gravel road, IMO. If your friend doesn't ride in the rain or on gravel, there isn't a great argument to convert.
So fat tires and fenders, that's the simple answer.
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Last edited by gugie; 08-12-17 at 11:24 PM.