My assessor is giving me spelling advice - but he's wrong
First of all: ask for a third opinion, just to be sure you are right.

Anyway, the nice thing about science people is that they are usually convinced by evidence, even when it goes against their previous thoughts. Write them a polite, respectful e-mail explaining why you think you are right, and pointing out reputable grammar sources to back you up. Show that you have done some additional research. There are good chances that this will work.