Duh! The fact he neglected to look into it is what makes it unreasonable. At minimum he should get a refund from this ***** not knowing what the hell's wrong with roofs. What else did he miss if he can discount this?
It's not his job to "look into" a roof. An inspector can't start removing shingles and digging into issues like that in order to see rotted wood. That's a roof contractor's job.
The inspector can only look at immediately visible things.
The inspector should have recommened a licensed roofer to come look at it. But in his opinion of the damage was that it did not appear to be a major issue from what he could see.
As I've stated a couple of times before, what the buyers do with that info is up to them. But me, I wouldn't take an inspectors word on something like this. He did his job once he pointed out/acknowledged that there was a roof stain and that it could have been caused by water. The recommendation of a roofer would have been nice, but ultimately, that's the buyers decision and responsibility. Home inspections are not home warranties.