Can someone explain this? Re child suppoort in the UK?

My friend got divorced last year – about September, I think. She is a working mother, earning a good salary. She’s single, with two children. She is the main custodian.
Her ex-husband has moved in with another single mother, a divorced woman with two children. She also works and receives maintenance for her two children.
My friend’s ex has approached the CSA to apply for a reduction in his child support, as he is now living with two children. They are not his own children, their father accepts his responsibility for them and pays his own child support.
My friend is in the process of losing 20% of her child support, which wasn’t terribly high, in the first place. As a lone parent, she pays the bills, mortgage, childcare, etc etc
Can someone please explain the logic behind this reduction. I’m not interested in opinions of the evilness of single mothers, I just want an explanation of why my friend’s children are now to be deprived of quite a large portion of their support.
This is a decision by the CSA. Nothing to do with lawyers.
If you check the website (I didn’t believe this either) you will see that this is so. CSA is now run by the Pensions and Works department.