Your friend could try the Amtrak through-service from Chicago's Union Station, which comprises a train from Union Stn to Milwaukee, then a Lamer's bus to U of W (Osh) Gruenhagen Conference Centre. Very convenient if he's staying at the uni dorms. Alternatively, there's a bus from KORD to Wittman Field (Bus Station) that originates from outside the KORD terminal building.

In 2019, our flight from ORD to ATW was cancelled due to thunderstorm activity and the next available United flight was four (4) days later. For my last two visits (2022 & 2023), I used the Amtrak option to mitigate the risk of cancelled flights into ATW. Fifty bucks, door to door, and he will not need a car.

HTH.