Is she overpaying council tax?

Hello! posting on behalf of a friend; My friend is a home owner and her property is currently council tax band c according to the council website and £1896.13 annually. Divided by 12, this should be should be £158.01 per month. As she is a single occupant, she should have had 25% account applied meaning the annual would come down to £1422.10 which divided by 12 is £118.50. She has been paying £142 per month as a single occupant which is £23.50 more than she should have been paying according to the calculations. How does she challenge this? All the calculations have been made using figures and information on the council and government website and simple maths! Any guidance is appreciated.