What, no recommendations of Toledo?

Seriously, though, I'll also echo the Ann Arbor suggestions, though a lot of the other recommendations (e.g. La Shish) are good as well. Perhaps a Tigers game if they're at home that day, or even a jaunt to Windsor (except for Friday and Saturday nights), if you're into seeing something a little different.

(If you go to Ann Arbor, consider a brief side trip to Hell.)

Also, as other people have mentioned, consider the distance between downtown Detroit, Windsor, and Ann Arbor to the airport. Depending on how early your friend's flight is, you may wish to consider airport-area accommodations. I tend to stay a few miles away if I have an early flight out (usually in Belleville), but the airport is in between everything, and not especially close to most of it.

Mike