No, she did not only eat junk. She ate a standard Western diet. Cereal with milk for breakfast, school dinners, tea which was sometimes home made spaghetti bol type meals and sometimes convenience foods like oven chips or chicken nuggets. It was exactly the same diet as any typical 21st century kid whose parents are working.
In order to get IBD, first off you have to be genetically susceptible. Most people aren't. My daughter's great granny had Crohn's (in middle age and until she died) so obviously we have the gene.
It is thought that this gene can be switched on by certain viral infections. Epstein Barr seems implicated as one of the "switch on" viruses. My DD tested positive to EB antibodies which means she was infected by this virus at some point in her childhood.
Then the situation is exacerbated by the way upf foods destroy the microbial balance of the intestine. As it happens I was on a permanent diet and we often used "low fat" spreads, "low sugar" biscuits etc. But even something as innocuous as instant gravy is choc full of maltodextrin, so if you have a lovely Sunday roast and pour bisto over it, you are impacting your microbiome.
The microbiome appears, in very simple terms, to effectively function as a protective layer over the inside of your intestine. So if you are killing that off, your inside protective layer is not working like it should. Add to that the inherited tendency to inflammation in the intestines that has been switched on by the virus, and you end up with inflammation and no protection so your insides get sores and ulcers on them. This turn stops you absorbing the goodness from your food and you get malnourished.
So MOST people can eat UPFs and just get a bit of a reduced microbiome, a bit of inflammation through the body, get a bit overweight, a bit of insulin resistance. But if you are unlucky and have the IBD gene, and the trigger virus, then the same foods have a much worse effect.
I know you are asking in good faith but it's a bit like asking a type 1 diabetic if they only ate sweets and sugar and that's why they are diabetic. No, it's a genetic lottery.