I am guessing this is an existing 3R 200a disco on a post, typical for manufactured homes and there should be a ground electrode system installed. I assume he wants to use one of those 100a temp power boxes that has a main breaker so this will just be a feeder tap situation. The only real issue is protection of the tap conductors and separating neutral and ground in the temp power panel. They are usually set up as a service (from my experience). He did say "SER" so I am guessing he knows that.
It sounds like a 240.21(B)(5) to me.