Assuming the premises of the situation, yes to your first question:
He may be argued into something that is not in his interest by the UFAI. (On the other hand, Rock-Placing Man evidently does not have a standard mental and physical architecture, so maybe he also happens to be immune to such attacks.)
The UFAI may take over his simulated universe and turn it into simulated paperclips.
Assuming the premises of the situation, yes to your first question:
He may be argued into something that is not in his interest by the UFAI. (On the other hand, Rock-Placing Man evidently does not have a standard mental and physical architecture, so maybe he also happens to be immune to such attacks.)
The UFAI may take over his simulated universe and turn it into simulated paperclips.
True, but it’s easy to deal with, just place one row of rocks differently and wipe the UFAI bugger out. Humans would not have this luxury.