If you work in Illinois and are a resident of Iowa, Kentucky, Michigan, or Wisconsin, you can submit exemption Form IL-W-5-NR to your employer.
https://www2.illinois.gov/rev/forms/withholding/Documents/currentyear/il-w-5-nr.pdf
However, if she is a contract employee (she gets a 1099-Misc, instead of a W-2), the rules may be different, if she works anytime in IL
