This is what I wrote:
Several shingles on the roof were seperated to inspect proper nailing. The areas that were checked found shingles were both nailed too high (which prevents securing the shingle below) or missing nails according to standard shingle fastening patterns.
Improper nailing can void the installation warranty and may create issues in the event of a storm related claim. While the roof system was performing we cannot gaurentee future issues will not arrise due to improper nailing observed in areas.
A roofing professional should review findings on the roof system to determine what, if any repairs can be made.
I report what I see and let the roofer make the call. Tearing it off would be the only real way to fix it, and I let the buyer know that he may be just fine for another 20 years, but I am not going to give it my blessing.
At the end of the day its wrong.