If you want to be up and above board about it I think you need to fill in a tax return and declare the income as a one-off payment. Might be worth ringing HMRC to discuss a 'hypothetical' situation where you planned to do some work outside of your employment. I imagine their website also has details about this. You certainly wouldn't need to set up a company or declare yourself self-employed. You've got ages to fill out a return BTW.

Another way of doing it is to ask the company she's done the work for if they will deduct tax and NI from the fee. I've done freelance work in the past where the company did all that for you (you had to specifically tell them you were freelance if you wanted to work out your own tax etc) and then sent you a proper payslip. Then you don't have to bother Mr Taxman at all.