Wow. Scared much? Posession of a cased, unloaded firearm, even on you person, for transport is perfectly legal in Chicago. Despite their best efforts, they have been unable to legally enforce their assault weapons ban, because of a legal injunction. The state level concealed carry law pre-empts Chicago and thus Chicago no longer has ANY regulations on possession of handguns (except the illegal carry of concealed, loaded, handguns, not being transported per state definition). The state regulates concealed carry.
http://www.isp.state.il.us/docs/transgun0-000.pdf
http://en.m.wikipedia.org/wiki/Gun_laws_in_Illinois
If your friend is from the US AND has a concealed carry license, any handguns can even be transported, loaded, while inside the car. Transportation has no legal definition, moving a firearm from place to place, anytime, to anywhere, is legal. As long as you are legally allowed to own the firearm. Transportation to, from, or around "restricted" places is also legal. As long as the firearm is being legally transported per the Illinois State Rules.
If your friend is stuck overnight in Chicago. It is perfectly legal to collect his bags and go to a hotel. It would even be legal to transport them via public transit, although I would probably take a cab or rent a car.
ETA: It would help to know what kind of firearm(s) your friend is transporting. There are Cook County ordinances for transportation that might be an issue for specific weapons (e.g., an AR15). But the long and short of it is your friend should unload, disassemble, and lock into a proper case, for transport. Separately, he should bring ammunition in a different case. That should keep him from running afoul of any transportation issues in Illinois.
-Rob