I thought I posted a reply yesterday but it's not here. Ordinarily
a Type 1 has a carbos-per-meal goal (90 grams for dinner in my case). So if I want that piece of pie that I know (or guess) is 70 grams carbo per slice, then I can:
a) cut back on the amount of carbo I eat in the rest of the meal (bread, flour, potatoes, starchy veg like carrots and turnips, grains and legumes, fruit, juice, etc.) so I still come close to my 90-gram target, or
b) eat what I want but calculate the total carbos, then either give myself extra insulin for the extra carbo or walk around the block a few times to burn off the extra. (Or, if I've eaten less than my target, give less insulin.)
The science shows that carbohydrates raise the blood sugar equally, regardless of source, although they raise it at different speeds (the whole glycemic index thing). So whether you consume 10 grams of carbo in the form of an apple, some pasta, or a chocolate fudge sundae, it's all the same to the body. The "old" thinking was that sugar raised the blood sugar more than fruit/veg/grain carbos, and therefore was to be avoided.
Most Type 1 diabetics can eat the same things as non-diabetics. We do not have to watch sugar or fat content any more than any other health-conscious person. Of course, all things in moderation.
BTW, my sympathies to your nephew on being diagnosed right before a major food-feast holiday. Trial by fire!