My buddy has a 2002 kawasaki prairie 300 and he needs stiffer suspension in the rear. He weighs about 380 lbs so every time he hits a bump its bottomed out and he has about 2 inches of travel as soon as he sits down. Is there a cheap fix for this, maybe an adjustable rear shock, like one off a dirtbike?

Check with Work Performance.
They have aftermarket shocks for some utility atvs
www.worksperformance.com