Fir my niece last time she was over she did mention that they never have dinner at her house. They have breakfast then late lunch as she call it (today was at 4pm as i was there).
So your DN only routinely gets two meals a day?! If she's having breakfast when she gets up, say 8am (since it's the holidays) then she's going 8 hours without a meal if she has "late lunch" at 4pm then potentially it's another 16 hours before she eats again, that is not normal or healthy, no wonder the poor child is looking for something to eat at 8pm!
I was all ready to say YABU to ignore a parent's instructioon not to feed their child after a certain time, as if I give instructions like that it's for a good reason (getting her to eat at "proper times", stop adding reasons for her to get out of bed, it giving her stomach aches to eat right before bed, medical reason etc) but reading your update I think YANBU.
I spent 10 years living in Spain before I came back to the UK 2 years ago, so my DD was brought up with Spanish mealtimes and it's something we still tend to do now especially at weekends and in holidays. So today DD8 had a glass of warm milk with a sprinkling of cereal in (she won't eat a whole bowl of cereal in the morning but doesn't like plain miilk so it's our compromise) when she got up at about 7am, then at about 11am we had tostada (toasted french bread slices with a little oil and tomato puree); lunch at 2pm was our big meal - noodle soup, couscous & roast chicken then yoghurt; At about 4pm she had a banana and then dinner at 7pm this evening was homemade butternut squash and sweet potato soup with a small roll.
My DD knows that she can help herself to fruit any time of the day but she usually asks me first anyway because she usually tries asking for chocolate/sweets first and then settles on fruit and she has a water bottle that she carries with her as she never remembers to get a drink if she's thirsty but if the water bottle is there, she will drink it. So generally she has 3 main meals and at least one snack a day ...if I started only feeding her twice a day she'd think her throat had been cut! Not to mention I can guarantee her blood sugar levels would be going mental and I'd have a grumpy child on my hands ... if it happened on a school day where she had to concentrate and work she would be having a meltdown long before 8pm!