Here's the answer to your question. I'll bet you dollars to donuts that your clients works for a company that is headquartered in Illinois. If so, that's how Illinois requires the W-2 to be reported - 100% Illinois wages and 100% Indiana wages. They made that legislative change several years which makes box 16 of the W-2 to look funky. 

Am I right? The employer on the W-2 in question is a company HEADQUARTERED in Illinois, yes?