Immigration wait time at ORD really is a lottery but the last couple of times I went through off the BA early flight the regular lines were very long (past the One-Stop). If he has no checked bags then it would appear that he can use the One-Stop which deals with immigration and customs in one step and is likely to be much quicker than the regular process. It is on the right hand side as you go down the corridor to immigration. Fortunately your friend is not trying to connect so it's just a matter of being prepared to stand in a long line.

Chicago is a great city to visit (I was there last weekend) so he should make the most of any free time.