i'd like to see proof of double butted being stronger.

anyway as far as your bike shop goes the torque thing is BS. he'd create way more torgue if he shifted onto the granny ring.

a decently built 36h may well be strong enough for him. but i'd go with a with a 48 tandem hub spaced for 135mm just to be sure. the extra 12 spokes and nipples weigh only a few ounces more, a worthwhile trade off for a wheel that'll be 33% stronger.