Why did he feel the need to change the sand in the first place? In general, normal use, sand shouldn't ever need replacement. So why? If the filter was ALREADY malfunctioning I'm guessing he blamed the wrong potential cause--the sand.
Did he use actual filter sand or sandbox, sand-blaster or mason's sand? If it's not filter sand you will get problems.
Did he use the correct amount for his filter? Too little or too much can create problems.
Did he check the hole-size on the laterals to ensure they weren't too big, that there weren't any cracks?
Is he sure his multi-valve is functioning correctly? A bad m-valve can cause all kinds of problems.