Child support past due but obligation is $0?

My boyfriend and I live in California. He has a 14 year old daughter, and the mother is just now deciding to come after him for child support because he wouldn't give her $200. He buys his daughter everything she has and anything she ever wants. The mother is a drunk, so he knew the money wasn't going to go to feeding his daughter. He has no problem paying child support, we even filled out all the paperwork and returned it in a timely manner, but now he is getting papers saying he owes past child support. All of the divorce and child support papers say that he is obligated to pay $0 in child and spousal support. Why is he getting papers now saying he owes over $47,000?