I have several issues with how child support is set up in most states, but I don't have an issue with the theory or concept behind child support. I think both parent share a financial responsibility for the child.
One thing I've never understood is why there isn't a standard child support rate per child. The needs of a child don't change per a parent's income, nor change as a man has more children. The way it is currently set up, a man pays a percentage of his gross income, but the percentage (at least in my state) decreases as the man has more kids. For instance, first kid is 30%, second is 25%, third plus is 20%, I believe. So the 3rd child should receive less support because of birth order? That's illogical.
As far as basing it off a man's income, that also seems inaccurate. I know some men who make pennies and pay $200/month in child support. Yes, $200. That is crazy. Regardless of income, the expense of raising a child doesn't change. I think the child support should be a standard amount, like $1,500/month (I don't have a kid so I'm not sure how reasonable this amount is). There are plenty of studies that show the average cost of raising a child. I think it should be based off that.
Then, to address the OP friend's point, having a set value would also address situations where women get windfalls in child support, which essentially becomes baby mama support, shopping habit support, etc...To account for the lifestyle that a child would assume if they lived in a dual income household, it would be reasonable to make the non custodial parent pay the set amount plus a percentage, like 5-10% of gross income , in addition to that set amount.
Having set child support amounts would make men and women think twice, in my non-child having opinion lol.