The firewall in the attic between these units is in place but the drywall is only on one side of the framing. Is this is a faulty installation or is this sufficient?
In this area current practice is a concrete block wall to the roof deck and at least 4’ of fire rated plywood on the roof deck on both sides of the wall as well as fire rated caulking at any gaps. I also see 5/8" drywall taped and finished on both sides with the fire rated plywood deck. You may want to fill out your profile and signature to get better advise for your area.
You can build a 1-hour fire separation assembly out of type X drywall and this can be done in a number of ways. For my part (as a generalist home inspector) I would simply note any visible defects (open holes, exposed joints) if any and move on.
Look it up.