So I'm 5'8" 205lbs athletic build with a bit of a beer gut. (You'll understand when ur 40!) with a size 9 boot. I ride a 159 all mountain east coast but I picked up a new stiff freeride board (166) for better pow float for my trips out west. I'm a little worried tho because the waist width is 25.7 and I barely get overhang with my stance set up. Should I return the board and drop down to a 162? The problem is the 162's max weight rating is 188lbs. Thoughts?
I wouldn't worry about the max weight, that's more of a guideline for the average person not really a rule like "The snowboard will break if you're too fat." FWIW I'm a little heavier than you, riding 156 or 157w in the park, you'll be fine despite the weight recommendation on a 162.
You do want some overhang, generally, otherwise you won't be able to properly pressure the edges/roll in to turns, etc.
This is true ^^^ but unless he's somehow guaranteed pow (I've been skunked in Whistler, Tahoe, Colorado, and SLC before...) or bringing both decks, he's gonna want a board that can negotiate the groomers, too