Did his employer withhold state taxes for WI? If so, then he would need to file a return for WI to get all of his taxes withheld back. Since WI and IL have reciprocal agreements, you only have to pay taxes to the state you live in, not the state you work in.
So, in this situation, if he did have WI taxes withheld, you would reduce his income for WI to $0, leaving the taxes withheld as they were. This will allow for a full refund of all taxes withheld.
You would then file your IL return and claim all income that he earned on the IL return.
OK so we have to file a state tax form for him in both states but the return would come from IL?
He would file in both states, a non-resident WI return and IL resident return. He would receive a refund from WI for taxes withheld.