From a reader:
I'm copy-editing a manuscript for another author and had a question. What do you think about using English spellings of words for writing that's meant for American readers?
If the writer is English and the work is specifically edited for American readers, he'll seem like a pretentious douche using England-English spellings.
So go right ahead and let him use them.
Just kidding! Change them. The spelling conventions should match the conventions of the readers. Otherwise, why bother with a separate edition?
Let's say the writer is an American writing for American readers. If for some reason he wants to use England-English spelling conventions, then he actually is a pretentious douche. Ignore his objections and change his spellings. And try to explain that his audience doesn't exist just to stroke his ego. Writing exists for the readers, not the other way around.