I think I finally got to the bottom of why my friend has such a large CSA contribution to make. The CSA calculated that he was working 40 hours per week in both his main and 2nd job. The 2nd job is actually only around 20 hours per week. My friend now says that he intends to appeal the assessment based on this.
However, my friend's ex has been making it difficult for him to see his boys, such that he is now lucky to see them one day a month. I pointed out that the previous assessment was based on the boys spending three nights a week with him, so won't he likely have to contribute even more if this is reassessed?