C.W.
Child support and custody are two different things. You should request a support change, is the financial statement for that?
I would put in stuff that is directly related to him: a car, rent/mortgage, electricity, phone. A negative may not raise flags but if it is WAYYY negative (like $400 or up) it may raise some flags. It would get as close to 0 left over as possible so that they can see that he does not pay support... just the fact that she lives with you it doesn't make sense to pay CHILD support to someone who doesn't have the child.