You're assuming she's working nine hours a day, five days a week and getting paid £20 for every single one of those hours, day in, day out - but that's not how it works.
When you're self-employed, you only get paid for the hours you're working. So travel time in between clients isn't paid time. Assuming an average of 10-15 minutes between clients, and four clients a day, that's an hour a day you're not being paid. So that's already £20 down per day. So £160 per day in earnings.
Then you have to put 20% of your earnings aside for tax and NI - so that's £36 into your savings, taking your earnings down to £124 a day.
Then you need to factor in your pension, your travel, your fuel, your vehicle maintenance and any supplies you use for work, as well as suitable clothing, any logo'd tops or signs, advertising etc.
Then the unpaid time you put in to your social media, your accounts and your diary every single week.
Then you need to factor in any time off you want to take, the time off your clients take when they're away or don't need you, or when you're sick or they're sick.
Suddenly that £20 an hour isn't looking that great after all. It's certainly nothing like 40k per year.
EDIT: maths!