What Fowler may have thought is irrelevant, what matters is what was actually meant and how the words are actually used.

The change since Fowler has been to use ‘will’ in place of ‘shall’, not the other way around. I have read more than Fowler on ‘will’ vs ‘shall’, but I've never read anything to suggest that any dialect uses ‘shall’ with the third person in a declarative statement to mean the simple future.

This is all only a minor point, since Harry didn't say ‘shall’ and it's clear what wedifrid meant. But I hope that it gets downvoted for unimportance rather than incorrectness. Some time a character may say ‘shall’ for a good reason.

ETA further clarification: My previous comment contains an element of saying that wedifrid used bad grammar. I stand by that, but I also accept the response (from your reply) that grammar usage varies and what wedifrid really meant is what matters. And that's why I wouldn't write a comment whose purpose was to say that wedifrid used bad grammar.

But my real purpose was to point out how, with a certain interpretation, ‘will’ and ‘shall’ give different meanings, which are just the meanings that people are debating as to what Harry meant: a factual claim that is (I admit) unjustified, or a declaration of intent that is (I would argue) justified. And the second meaning could be what Harry meant, if he used bad (or at least unnecessarily ambiguous) grammar.