A friend of mine is looking at a boat with twin 350mags with 300 hours. What should he be looking for?

Take a look at the exhaust manifold to riser joint. If you see any signs of weeping (the riser gaskets are prone to failure) you can be sure they are leaking internally also and it will need a valve job. If no signs of leaking, they have plenty of life left with only 300 hrs.

My buddy's dad has a 20' Formula with a 350 Mag. He has 1300+ hrs on it. Still runs strong and never even had a valve cover removed.

700 on mine NO problems whatsoever