Okay, so obviously, we know that ASD food issues and medical conditions DO exist.
But toddlers also commonly go through finicky and unreasonable periods, which parents can make worse by taking too seriously, either anxiously over-catering or forcefully demanding the child comply.
I'm older and personally, the "finicky" scenario is one I've seen far more frequently than some kind of true medical issue. (I also raised a kid who is on the autism spectrum and had absolutely NO food issues. So just to note, ASD related food issues would only apply to a subset of children with ASD).
And of course, most of us probably would prefer "beige food" and chips and juice, to the whole foods that are nutritious. I don't know about the UK but in the US, fully 70% of adults are overweight and the health issues from it are staggering. There are many people out there who get used to that kind of poor nutrition from the start. That exists too.
And of course, either way, the parents usually don't like being contradicted, which may be because they know best but may also be because they just think they do.
But yeah, if it's somebody else's kid, probably best to stay out of it, unless it seems dangerous OR you're in a position with them where speaking your mind is allowable, as a sister might be. (However, OP has said she didn't say anything to her sister).