A guest asks for ketchup with the tasting menu. Whose meal is it?

Friday service just finished, so this is fresh.

We run a nine-course tasting menu on weekends. Tonight a guest at table six asked for ketchup with the fourth course, a piece of hamour with a sauce my sous-chef spent two days getting right. The server came back to the pass to ask me what to do.

Two views fought in my head:

• The chef's view. A tasting menu is composed, like a piece of music. Each course is balanced against the one before. Ketchup doesn't just change his dish, it changes what the next three courses taste like. You wouldn't ask a pianist to skip the slow movement.
• The host's view. He is paying, and it's his evening. Brillat-Savarin wrote that to invite someone is to take charge of his happiness for as long as he is under your roof. Not your happiness. His.

We sent the ketchup, in a small bowl, with no comment. I'm still not sure whether that was hospitality or giving up.

So: is a dish a work, like a painting the guest should take as it is, or a service, which exists for the person eating? Is refusing the ketchup snobbery, or protecting him from a worse evening than he'd get if he trusted us? And does Hume's idea that some palates are better than others give me any right to overrule his?