I agree, you will need to get authorization to do this from a person at ORD. They can delete that segment and then he can drive to the destination. If the flight is a UX flight and keeps getting pushed back chances are it wont go at all as the plane will probably need to be in ORD tomorrow morning.

But if he just leaves, the return will be cancelled. It may be quicker for him to go to the ticket counter than the CSR as the lines will be longer in the terminal.