He has killed nice mule deer in the past but never a true big mountain muley
Maybe not a "true big mountain muley" in your opinion but they are great deer in my opinion for bow hunting during general season. I like everyone else has seen bigger on the hoof and every year I think man if I had a gun I could shoot that 175-180 deer, they get big by staying out of bow range during the bow season and out of sight during rifle. Rifle tag in the late season is a great way to score a big buck in the 170 range. I am not sure what you consider a "true big mountain muley" but I don't recall seeing more than a few over 170 from last years hunts.
Your buddy should have a great time and see plenty of deer if the snow is there and hopefully will score what he considers a "true big mountain muley".