What do you do when custodial parent will not submit any income for purposes of needing a modification?

My husband needs to modify child support , the amount is literally 50% to much, the custodial parent refuses to give any income and has reported there is none to my husband, Need to do this ASAP . Is there a way to get that info . or would he just put 0 on her income . Also with 0 income does she still get credit for support of other children and insurance?