Nothing lives in Illinois, it is a sterile wasteland.
Ok, just kidding.
He should make it 4' deep if he can and make sure to maintain an ice free area with a bubbler during winter. Very few fish are imported that survive temps so low while adapting to the higher summer temps. If the pond is large enough (at least 4'D x 8'W x 12'L) he could go with sterlets and add a couple of the less common natives such as pike minnows and this one the Highfin carpsucker (Carpoides velifer), an attractive fish that can reach 20".
Minimum requirement is 2.5 feet deep. I know a guy in Minnesotta that keeps koi in his pond all year long. Talk about cold... I thought Chicago was bad...