As far as fletching length is concerned, the larger the diameter of the arrow shaft, the longer the feather it takes to stabilize the arrow. So, generally, with aluminum shafts, I like to use 5" feathers, with the exception of maybe 2016s or 1916s. On these, along with micro-diameter carbon shafts, you can very often get by with 4" feathers.
I, too, think your buddy would benefit more from using arrows closer to the weight of your arrows than the ones he's using now.
I myself am using a 557 gr. arrow and pulling around 51# with my Beeler longbow and am very satisfied with the speed and penetration of my arrows.