I don't think the above answer was a smart ars* answer, I would also be surprised if she earned enough to pay tax.
Anyway, as she is working whilst still at school, then by law she is not allowed to work more than 2 hours on a Sunday, so her manager is right - see this and pay particular attention to the last paragraph:
Working hours
There are rules that regulate what times of the day you can work and for how long. These are different depending on your age.
14 year olds
There are a lot of rules that control working hours of children, but the basic ones are:
* during term time, you can only work for two hours on weekdays and Sundays
* during term time, you can only work for five hours on Saturdays
* during a school holiday, you can work for up to five hours on a week day or a Saturday
* during a school holiday, you can't work for more than two hours on a Sunday
* you cannot work before 7.00 am or after 7.00 pm on any day
15 and 16 year olds
If you're 15 or 16 and are working while you're still at school, your rights are almost identical to those of 14 year olds. However, you are allowed to work for up to eight hours on Saturdays or during the school holidays.