Ladder stands are not permanent, so is it legal? Beats me, depends on where you are, some places you have to take em all out as you hunt, some have to be out by a certain date, some don't have rules.
Would I hunt in that stand, heck no! Wouldn't feel right, but also, how do I know he set up the stand safely, didn't do something else to it etc.
I would vacate the area the first time, but I might do something to tell me if he's actively hunting it, something harmless, stick across the steps etc. If it's still there a week later I wouldn't hesitate to hunt there, but not in his stand. It's public land, you can't "reserve" it. If I was hunting it and he came along and caused a fuss, I'd probably move on, life's to short to get in stupid fights, I'd have more than 1 spot picked on any piece of property anyway.