Is this normal sauna behaviour?

Dry brushing is to encourage lymphatic drainage. Exfoliating is a very different thing. Personally I would not be bothered by dry brushing. Exfoliating, yes. I wonder if OP is confused.

Yeah no one cares about someone wrapping a clean towel around them in sauna, it's pretty common for teen girls to do when they're in the middle of puberty and no one judges them, we know they'll grow out of it in a few years.

A perfectly clean swimming costume would be somewhat less vile because, as you say, you’re not bringing the swimming pool chlorine into the sauna, but in practice most people would not be that diligent, and it would be even more vile to then go swimming afterwards in a costume full of sweat. I just don’t get the insistence on sweating into clothing. For many people outside the Anglophone world it’s a hygiene faux-pas on a level with picking your nose or coughing into someone’s face.
You have to take a towel into a dry (Finnish) sauna anyway, so I don’t think anyone would have an issue with you wrapping a second towel round your boobs and body too and sitting like that, if you really can’t face the nudity. This is very much about avoiding cross-contamination and ensuring good hygiene rather than nudity for its own sake. When you come out of the sauna you shower all the sweat away - it’s horrible to think of that sweat being trapped in clothing instead. As the Germans say, igitigit.

The word is puerile for excessive prudishness around the human body. I‘m not sure what purile means (sorry, picking up on a typo is so puerile, no?).
Prudes do love the word flaunting, don‘t they! You can‘t just be naked, you have to flaunt it. Breastfeeding is apparently only ok if you don‘t flash your boobs. It‘s a deeply weird neurosis, and good to hear that there‘s at least one sauna in the UK where common sense prevails.
I really don‘t think Germans are terribly ‘liberated’ in lots of respects, they‘re just not uptight about being naked where that is the most hygienic option - the national uptightedness / neurosis is reserved for many other things instead 😁.
There was a thread recently about a woman sitting nude on a changing room bench and how icky that was, and so many people were clearly unable to distinguish between ‘eww she‘s naked‘ and the lack of hygiene in not sitting on a towel. So the stereotype is sadly anything but ‘old’. And it’s very damaging if you’re perpetuating this tripe for your DC.

If it was a clean swimming costume it wouldn't be vile (just weird because it'd be like wearing a swimming costume in the bath and probably uncomfortable in the heat) but if you've been in a pool in it then it will release chlorine in the heat which is bad to breathe in and can affect asthmatics.

They're just about to open a sauna in my local park. I wasn't keen on going before, but after reading this I feel like mounting a protest outside

Excuse my ignorance, why is it vile to wear a swimsuit in a sauna? (Non sauna user)

That‘s really good to hear, ludn! Wearing a swimming costume in the sauna is utterly vile, TBH.

I don't think that's the case. In my absolute backwater of a town in the UK the sauna in the nice local hotel spa of which I am member frequently has lots of nude people in it and no one bats an eyelid.

I do take exception to the person who sits in it with a head torch on to read her book though.

That's vasta, a bundle of leafy birch twigs, though fresh is better than dried (if dried you need to soak them first) but that's something you tend to do in your own sauna, though some traditional lakeside saunas might allow it but not a gym sauna.

The stereotyping of British as prudish around nudity is boring and old to be honest. People decide what they are comfortable with around nudity. There's plenty of non British people who have their own individual boundaries in this area. It's the flaunting 'look how liberated we are and how prudish you are British people' that's purile.

Dry brushing is for opening pores and lymphatic drainage. Doing it before a sauna is probably the best time to do it. It's not purely exfoliation. If she did it on a towel can't see why it's an issue.

It would certainly not be acceptable in any dry sauna I‘ve been to in Germany or Austria. I‘ve never seen a specific rule against it, but I think that’s because it just hasn’t occurred to anyone to do something as unhygenic as brush dead skin around in a public setting.
On a different note, it’s always good fun to witness prudish Brits get their knickers in a twist, as it were, about any form of nudity. It’s utterly puerile.

Assuming it was a Turkish (wet) sauna and there was a hose where you are normally expected to clean your place when you finish anyway, I don't think there was a problem - Turkish saunas often provide salt for this purpose anyway.
Finnish saunas are dry and much hotter, I never saw anyone trying to exfoliate in a Finnish sauna, presumably you would do yourself an injury....?

But in your position, I would ask your local friends what they think, rather than Mumsnet where it's mostly Brits who are not familiar with any kind of sauna etiquette and will be lost after the word 'naked' anyway.

there are dead skin from other people everywhere. I wouldn’t care

I think it’s normal in some places, I know rubbing with twigs is

Not good! But over the past month, l’ve witnessed someone eating in a sauna and someone else watching videos on her phone with the volume on max 😤.

I remember an item where in Scandinavian countries you had your skin beaten with dried branches or twigs before plunging into icy water.

Yeah I'm in Finland and if you did this in a public sauna you'd get comments for sure. If there's an old lady in, you'd get told to stop (they are the enforcers of the rules - a friend got told off by one for wearing her swimming costume in the shower, as you have to shower naked)

This would not be ok in any Finnish public sauna I've been in!

seems fine to me

No, it says in the OP that it's not in the UK.

Scraping the dead skin off is normal in sauna (feel sooo much cleaner after!) but that's what you do in your own private sauna, not in a public sauna.

As others have said, if she rinsed down after I wouldn’t have a problem. I’ve practically had my epidermis loofer’d off in a Turkish sauna before.