She needs to contact her tax office and they'll issue a further revised tax code if necessary.
BR is common for people with second jobs so perhaps HMRC believes (rightly or wrongly) that the employee has other income elsewhere that uses up their tax free pay. IF HMRC is wrong about that then it is a matter between HMRC and the employee to sort out.
I thought that was the case as I too have a 2nd job and am taxed at BR. I'll double check with her today but as far as i know she hasn't worked full time for about 14 years.