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Vibration... two different kinds of vibration. Mechanical vibration is the obvious one, if the filament happens to be the epicenter of vibration at cruising speed, it will fatigue the filament and fracture it. The other kind of vibration is electrical... if the system has borderline troublesome connections, switches, grounds, the voltage to the bulb can fluctuate, imperceptibly even, but enough that the heating/cooling cycles of the filament will fatigue it and cause it to break.
Next time tell him to head for the local truck stop and get a heavy duty truck bulb.
They tend to hold up to vibration a bit better. Check fasteners everywhere, check the headlamp shell for cracks, maybe new handlebar bushings are in order?
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