I thought you didn't know anything about her tax affairs? You now seem confident that she's on HMRC's radar.
It didn't go without saying that she needs to absorb the cost of her travel time. You said that you paid her a fee for 20 minutes; she works for longer than that to provide you with an appointment, so can't as you imply provide 3 x 20mins slots in an hour.
And yes, having done some non-client based work with people working in a similar occupation (although with a lot more experience) I do find it hard to believe that someone working in a hair salon five days a week and cash in hand/sole trader one day a week is making 'a lot of money' in real terms ie the current cost of housing.
Even if this young woman fits in 20 appointments on her self-employed day, so works at least a 10 hour day (depending on how much travel she has do to) 52 weeks a year, and offsetting the cost of products, equipment, training, petrol/transport and admin, she'll be pocketing an additional couple of grand a month, which isn't a lot in a Tory Britain, I'm afraid.
If she wants to go on holiday, has a family emergency or becomes injured or ill, that will obviously decline considerably.
As I said earlier, I don't have a problem with young people working hard. I did something similarly entrepreneurial when I was younger (though I have always paid taxes due), but as I was born in more benovelent times, I worked to finance myself through 'A' levels, received a full grant for HE and entered the housing market when it was saner.
I wish her and all young people lots of good fortune. And boy oh boy they're going to need it when they truly realise what Johnson has in store for us all.