I personally know at least 3 situations where Dad has custody now and asks no child support from Mom. I only know one custodial Mom who asks no child support from Dad.
The one Mom was a single woman who wanted a baby. She made the agreement that if he agreed to father the baby she would never ask child support and he would still get to see the child. She kept her word and their son is almost grown now.
All the Dads in these situations gained custody by Mom allowing them to take custody. If you can, it is often better to just keep the tax deductions and not create the conflict that comes with a child support dispute and all of the damage that conflict can do to the kids.
A woman who truely makes 100,000 probobly does not need the child support money to make ends meet. She is just punishing him and probobly enjoys it. A little side hobby.
Is she legally entitled to the money...sure. Does our child support system often not consider all the relevant factors and make unfair CS decisions that severely detriment the non-custodial parent...definately. Is he probobly being asked to pay too much...probobly.