They need to install a new ground wire from the fixture's bonding point to the box.

If you HAVE a 3-wire nranch circuit (hot-neutral-ground) the fixture should be grounded by attaching to the grounded box or the ground wire itself.

If you have an older circuit that is 2-wire (hot-neutral) it is not really going to make any difference though, you do not have a ground anyway.

Note that some wiring methods (AC cable, metal conduit) use the armor or conduit as the ground connection and not a separate wire.

Are we speaking of a chandelier? Regardless, if the ground wire (I assume green) is visible to the casual observer from 18ft.below, I would ask the GC to come back and properly ground the fixture.

I think he's talking about a ground wire for the FIXTURE not one feeding the box.

A fixture ground wire MUST be connected to a viable ground regardless of what type of box is used.

I've never seen a plastic box in the US with a grounding screw. I've seen some in Canada that have the screw solely because they have a metal strap that runs from there up to the device screws to handle devices that are grounded that way.