This is a loaded subject with lots of pitfalls. basically anything that benefits the child can be considered child support. Food, clothing, shelter (rent or mortgage), utilities, car payments (you drive the child in the car), car insurance (needed to be able to drive the car), gas, doctor, dentist. That being said if she has no receipts to prove the actual cost she has the burden to prove it is legitimate. And she should be consulting the father before she spends large sums of money.
I strongly suggest that you buy two composition notebooks like you used in school. In one keep a log of EVERYTHING you pay for and EVERYTHING she requests reimbursement for. You have a right to request proof of the expenses. And keep your receipts. In the other notebook keep a journal of ALL time spent with the child and phone calls too.
These could be invaluable if she ever tries to claim that dad is not paying his fair share or not exercising his visitation or if she goes back to court to change the custody or support order. I have a friend who did this from the time of her divorce when her son was age 8. At age 12 she went back in for an increase in support, teenagers are more expensive and the cost of living has increased in 4 years. Dad tried to claim he saw the child all the time but she could prove exactly when he saw his son and even when she had driven the son to Valencia after dad moved to Bakersfield to make visitation easier for dad. All I can say is document, document, document. It may seem petty but consider it an insurance policy. The reason you use composition notebooks is because it is easy to tell if pages have been torn out.