ejector pump venting
That plumber is full of it. He's doing what's called "wet venting". That involves using a pipe that drains water/effluent as a drain and a vent. Your drain pipes are never full of water...The water drains down them in a cylindrical cross section, leaving room for air. Wet venting is ok under certian circumstances for venting basic household plumbing fixtures.
The problem with wet venting the ejector crock is that it is a powered discharge of water, not a gravity drain. Therefore it demands a lot more air than a p-trap (for instance) would require.
Furthermore, I'd call the builder back and tell him that the plumber's name wasn't on your contract, his was. He needs to deal with his subcontractor to make this right and not pawn it off on you, the customer.