Most of these modern Japanese bikes a really set up for 2 different people at the same time. The long travel suspension makes touching the ground less than easy even for people of average or even a little above average height while the taller person touches the ground nicely. Yet while riding it is reversed, the average to taller person is cramped and the average and slightly shorter person fits in there like a glove.
At 5'8 i'm not much taller than your friend and i've been riding these long travel mx bikes for the last 23 years and have never lowered them or anything. My current bike is an 06 KXF 250 which is the best bike I've ever ridden.
If your buddy wants to do a little something, have him get a turkey knife and shave the seat down a little and put the seat cover back on.