I get that, chipboard on the outside would annoy me too!I don’t think it’s unreasonable at all. When you’re paying a shed load you want it done right. My husband and father in law fitted our kitchen. They fitted one bag panel of one cupboard the wrong way round (so it shows chipboard rather than white) and it drives me mad. I’d get it changed if I were Katie too! And she may not have known it was different inside until it was fitted, as it’s presumably all “flat pack” and put together by their joiner
I do see everyone's point about changing it, I just still think she must come across as a pain in the arse to their builders