In Michigan on public land he would have been legal.
A butthead, perhaps, but still legal.
I would at least have let him know--quietly--that he was in my stand by walking up underneath him, and letting him make the next move.
If he's got any sort of conscience, he'd get down and leave or be polite.
If he was an ass, I'd leave and call the DNR or local cops, and let them know of a pending incident if the guy was unruly or threatening.
I did find that someone used my stand last year, and the only thing they did was leave the seat down and not tie off my bow rope. No biggie...but hey, I get ALL TYPES on my public land area...some of which are tree-huggers who argue with me that hunting AT ALL is illegal on public land...!!
As Forrest said: "Stupid is as Stupid does..."