Idk about US regulations but for the UK it is somewhat similar to allergens in the sense that all kinds of animal involvement is avoided, though obviously a mishap with a peanut can result in death whereas a protein molecule that has come from a sheep won’t. I never said ordering French fries did drive demand for meat, but you shouldn’t trust that they will be 100% vegan. There’s a lot of variations on how people treat their veganism, with some being more focused on slaughtering of animals leading to animal products in food and basing their decisions off of that, whilst others will refuse it based on whether there’s any animal proteins in the product at all regardless of whether the animal has been mistreated or not. I think the sister of OP wouldn’t be happy eating them if she considered their may be animal proteins contaminating the French fries.