The chap was working on a rig as I understand it, therefore he is likely to have a significant income. The CSA will charge 20% of his net income (after tax) per month, this will reduce slightly if Mum has another child. Therefore, £1,000 per month in my view is not excessive given I know what sort of salaries get paid on the rigs.
And I've seen first hand how a woman will use her children to have a go at a non resident parent. I've seen children being sent without coats, shoes that are totally inappropriate or have holed right through, trousers with gaping holes in them, frayed hems etc. Talking to the ex won't work (tried that). The easiest way to deal with it as I have learned is to simply ignore it and rise above it as Vibra said. It is done to "press buttons". As has been said, buy the child appropriate clothing so that she is always warm and comfortable and don't sink to her level.
This actually mirrors some of the stuff I have on a daily basis with my FH so I have incredible sympathy with the brother. I still cannot understand how a mother can treat a child in this way despite seeing it first hand.