Hopefully common sense will prevail before any lawyers are inserted into the process. With all the cable and recessed fixtures in place at the time of HVAC roughin, the HVAC master should have been able to evaluate the situation and install accordingly, or at least, call the GC and start a change order for the "increased cost of ducting the return".
I'll just bet that the HVAC master stayed in the shop and sent out his lowest priced installer to handle the "simple new construction job", and that the installer just didn't give it any thought. Well. . .other than "hurry up and get out of here."