Harry Potter, Summer before 4th year

The school nurse had seen what Aunt Petunia’s eyes — so sharp when it came to spotting fingerprints on her gleaming walls, and in observing the comings and goings of the neighbors — simply refused to see: that far from needing extra nourishment, Dudley had reached roughly the size and weight of a young killer whale.

So — after many tantrums, after arguments that shook Harry’s bedroom floor, and many tears from Aunt Petunia — the new regime had begun. The diet sheet that had been sent by the Smeltings school nurse had been taped to the fridge, which had been emptied of all Dudley’s favorite things — fizzy drinks and cakes, chocolate bars and burgers — and filled instead with fruit and vegetables and the sorts of things that Uncle Vernon called “rabbit food.”
To make Dudley feel better about it all, Aunt Petunia had insisted that the whole family follow the diet too. Rowling, J.K. Harry Potter and the Goblet of Fire (p. 11). Pottermore Publishing. Kindle Edition.

In return, Harry had two notes from Sirius, one for the bank to audit his accounts and release funds to pay for his seeing a specialist in teenage growth and dietary needs. That was well known in both worlds. And the other to the Dursleys letting them know he was keeping an eye on them and the amount of money they were being paid from the Potter accounts for his godson’s care. And having those accounts audited. So, they had better have the receipts to show the auditor for the last five years at least.
Oh, and by the way, his godson was going to be seeing a health specialist and his own dietician about his dietary needs. Be dears and follow his diet as well as they were following their son’s new diet and exercise needs.

It was personally delivered by the muggleborn auditor the bank used for occasions such as this. Who arrived with his bruiser of a back-up, and a demand that the last year’s receipts be on the table for his audit within the next fifteen minutes.