From what I can see the Xterra in all models is listed as taller than the Montero, and only weight about 10% less, the other factors I cant comment on.
All that I was trying to allude to, as well as others who differ from your opinion, is that just because you switch your tires from 265 70R16 to 235 85R16 it doesn't meant that all of sudden his Montero will turn into complete disaster on the road and be dangerous and rolling over on every corner.
Will he notice a difference, of course, they are different sized tires, but you just adjust to the new tires, notice the limitations and move on. It isn`t quite nearly as dramatic as you suggest.
In the end run what you feel is safe, and what you think looks good, everyone else`s opinion is just that