My wifes grandad bought a zero turn and the belt falls off every time we cut?

Great piece of machinery, but not satisfied with belt issues. Please help?

Cub made a lot of ZTR's, feel like narowing down the field a bit ?
Was it new or used ?
was it yesterday, last week. last month , last year ?

There was a design problem with Cub deck from while back that was fixed by fitting a bigger idler pulley anchored in a different position.