Child Support Agency (CSA)? How does it work? A friend of mine is in a right state...?

She is a single woman at the end of her tether. They have a shared reponsibility for their son. Her ex (the childs father, obviously) moved on and is living with someone else and they both work. Not sure if THEY have kids together. But ex is effectively bullying my friend into paying extra for things. Like "give me 20 quid for xxxxxxxxxx". She has said that if her son wants things then she will certainly pay for him to have them but not give him cash in hand, so to speak. Where does she stand?