My best guess is that his clutches are out of allignment or are in desperate need of cleaning. Do his belts smoke before they break or just come apart? If they are smoking, i would also check the track tension.
Check the engine mounts. A friend of mine had a Cat ZR600 something or another and his engine would twist under power and he would lose the belts in less than 200 miles. He put in some kind of aftermarket torque limiter deal that somewhat solved the problem. He sold the sled after one season so I don't know if he ever really got the problem fixed correctly.
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