FWIW I HATED helmets as a kid, young adult, and even today, it's still a problem. I was a serious long distance biker when I was a teenager, so it really scared my parents that I never wore a helmet. But I survived (sigh).
I have thick hair and there's never enough cooling so I'd get very claustrophobic wearing one. I also felt as if they threw off my balance and hastened a nasty crick in my neck. So yay.
But I haven't biked since I moved here and probably won't just due to safety issues. If I do, I'll suck it up and wear mine.
But the real problem I've always had fitting a helmet is the shape. Usually, they're made in a circle. My head is more oval and I could feel it crushing against my forhead and just above my spine in the back. Since the pressure points were always on the front & rear, unless they were stuffed with padding or tight on the chin strap, they'd tip from side to side like a rowboat. Yuck. Thankfully, there's more of a selection these days and I have on that fits the shape of my oval head.
Different helmets have different shapes and that's what you have to look at to find the right fit. There should be room to grow, but the space should have some sort of padding that velcros to fill the current gaps. Center on forehead, not pushed back on the head. Straps are a pain to get right, but she needs to be able to open her jaw comfortably and turn her head easily. If it's set correctly on her head, they'll exist like a pre-crash pretensioner.
Sometimes a bike shop or internet will provide a better selection. A bike shop should have someone showing how it's supposed to fit correctly too.
An illfitting helmet is torture.
I ended up ordering dd's online. I can't remember the manufacturer off hand, but if she's in a fall/collision with something or when she outgrows it, they'll exchange it for credit on a new brain bucket.
hth with some suggestions, but seriously, listen to her opinion. If it fits properly, it will be comfortable and she'll wear it correctly even out of your sight.