Ah - I was this girl Camelia3.
I once went, aged 16, to visit my boyfriend's family at his home, where I stayed for four days. I bathed a couple of days before I went and washed my hair. Before bedtime his mum invited me to have a bath and didn't disguise her shock when I told her that I'd 'had one already this week'. In fact her response got me thinking, but it was a long time before I started to shower/bathe with any regularity. I also didn't have a toothbrush until I was fourteen, so brushing my teeth is something that I had to (eventually) force myself to do until it became a habit.
My advice would be to let this girl find her own way, but there are ways you could help her along the path. Does she swim? If so then arrange a weekend trip to the local pool, give her new toiletries to use in the shower. Ask her for feedback on the toiletries you buy her. Talk to her about when you were younger and what you got up to - and you can slip in information about how you and your friends managed hygiene, periods, sanitary towels etc. Hopefully your conversations can be used by both of you as a prompt to share more about life in general and build a closer relationship.
My foster children had very different cultural norms to me, but began to make some changes as they watched and listened and engaged with me, but I never undermined their own ways of doing things either.