I did the same thing for my friends daughter of the same age few years back, I found her a clerical data inputting job working in the office my friends recruitment agency.
They naturally knew employment law far better than me (it was not the first time for them) and there were no problems at all, even though she only worked less than 2 weeks (another story!) she was able to be paid gross without deductions or having to wait for a rebate.
She actually earned less than P&O robbed me for her ferry tickets (it came at the worst time after the loss of duty free) which she wanted to pay for but I managed to fob her off that I had built up loyalty points and not to worry.
I see no obstacle at all but as your friend what documantation she should bring with her to be paid gross if applicable.
Unless times have changed I guess she/ her employer signed a students exemtion certificate:
http://www.studentgems.com/useful_stuff_student_financial.asp
It looks like form 38 S is the correct one.
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