Rather than let her pay less she should be charged the regular amount with a debt accumulating for unpaid fees (the dad's portion). That will be needed in court to prove that his contribution is needed.

Be aware that the family will likely leave as soon as a subsidized spot is available so cutting some slack till she finds cheaper care is one option if you want to help out and it gives time to replace. As far as doing it on a long term basis her situation isn't your fault either and you should not be made to suffer.