You have two reasons for replacing the plywood. One is the structural failure you see in the de-lamination. The other is that there is undoubtedly mold damage and mold spores in not only the sheathing but in other places also. Has your inspector raised this concern? I would expect from seeing this problem a dozen times ( why don't people hook up their vents and kleep them clean?) that you have mold problems in the attic insulation as well. You need to be sure that the vent is correctly installed, and ANY material harboring mold is replaced with new.
As to the structural issues, There may be local jurisdictions - mostly in seismic zones and hurricane zones that would have specific issues with nailing new over old that would probably be adressed with increasing the number of fasteners to overcome the lost shear strenth in offsetting materials like that. Other places might reject that option outright, requiring direct application to satisfy requirements for shear.
Here is another hint for your negotiations, If the current roof sheathing has only 3/8", they may be considering the same for replacement. that is still acepted in some places but is woefully inadequate for roof loads if you still want it to look good in a few years to avoid sagging between rafters. depending on the area of the country you are in, you would want 1/2" or 5/8" sheathing
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