My questions would be this:
In most cases if the physical inspection reveals an additional problem, the inspector suggests that you obtain an inspection from a qualified expert, such as a roofing contractor.
You will have to pay for the inspection, but if it were me, I'd want to know, IN DETAIL, the exact condition of the roof. I would not rely on a physical inspector's report solely.
Once the roofing contractor has submitted a report, THEN you may have grounds to negotiate an allowance to repair/replace the roof.