You said it not likely to be ND cos she's not shown signs previously
I'd still say don't dismiss it as when puberty creeps closer, hormones changing and new brain development being ramped up to keep up with these changes CAN make you act differently than before
I'm currently reading a lovely book by Lisa Damour about teenage girls and she goes in depth about this sort of stuff, worth a read. It isn't long either, I'm on my second read
Some girls can't face their bodies changing and go into denial. All help is brushed off and ignored, slowly spiralling into a despair that they cannot communicate or know how to get out of
Gentle indirect communication could help, if there is an older girl in the family that could help her is she's not listening to either of you
It's tough to tackle as too much talk about this and invites to spas will just send her running for the hills.
She's embarrassed and her father shrugging about it won't help her self esteem, and as some have said, could lead to bullying
I personally struggled w self care when entering puberty, I felt out of control, scared and too embarrassed to even talk to my mum
Only as an adult I realised that a bit thing for me was hating shower, too cold before you go in, too noisy, half your body is wet and cold while the other is drenched under the shower.
Baths are soothing and relaxing. Even if she's just soaking in it that's a start, maybe run bubbles so there's soap in the water.
Take the pressure off and leave her in there as along as she wants and don't ask about whether she washed herself or to hurry up
This is crucial time where she will be seeking personal time and space
Also
Maybe introduce dry shampoo. Real handy and gets rid of the grease, maybe also treat her to a teen friendly perfume
The rest will follow!