If it is indeed her sole income then a call to HMRC is needed here. They are taxing this income at basic rate ie no personal allowance. As the previous respondent stated, this is what happens when your allowances are given against another income.
Is your Mum able to call and speak to them? If she'd rather you help, she can go through the initial security questions then tell them you have her authority to speak about these matters for her.
0300 200 3300 quoting her NI number.