Rude? No, I don't think it's rude. If someone else at the table made a comment about it, that would be rude. If anyone at the table farted loudly and then said "cor, blimey, that was a hot one!", that would be rude. If the waiter suggested someone had the fruit salad instead of chocolate cake so they didn't get any fatter, that would be rude.
But personally, I don't think someone ordering a course that is on the menu, even if no one else in their party does, is particularly rude.