Does he have to do anything else?
I don't think you need to do the Scarab Conga line and you don't need to make 9 Scarab bases a turn. I found 2 squads of 10 came in at a fair price backed by the ability to make 2 more a turn didn't really cost me too much and added me lots of flexibilty.
Relying on just scarabs and spyders might not be viable when the cat gets out of the bag, but who knows? I really think Immotek makes the list and it seems like he'll work with almost any Necron army.