“ My question is how can he owe so much to MO when he hasn't worked here or received checks from here?’---> The person’s home state is MO. And the person , as a nonresident of IL, works and earns income and files IL tax return as a nonresident of IL( since there is No Reciprocal State Tax Agreement between MO and IL) to IL.The person can claim his IL taxes paid to IL on his MO tax return. The person is liable for MO taxes on his bonus check income and IL income. However, as said, the person can deduct IL taxes on hid MO return as state tax credit.
“ Is this a mistake and he should get a second look or if this is right, how can it be?”---->I do not think so.