It will not effect her State pension because it is paid free of tax. However, it forms part of her taxable income. So if she has an income (including the State pension) of more than £9,440 for the year 2013-14, then income tax at 20% will be payable on the amount over that level. For example, if the has a full basic State pension (which is about £110 per week or £5,720 per annum) she can earn £3,720 (about £71 per week) before tax is due.