From his house, no I don't think a consumer mortar could hit a plane. The planes weren't that low.
But the area surrounding the airport is very densely populated. Given all those people, I'm not surprised some dumb kid (or adult) lit fireworks near the airport.
Also, most fireworks are illegal in Illinois. But it's only a 30 minute drive from Chicago to Indiana.
There is a fireworks store near me in Pennsylvania that is off I-95 at maybe the first exit over the Delaware line. When you enter the building, you go into an atrium with two doors that go into two separate sub-stores divided by a wall. In the atrium, they check your driver's license. If you have a Pennsylvania license, you have to enter the store that only sells non-explosive, non-aerial fireworks (no rockets, mortars, firecrackers, Roman candles, etc). If you have a Delaware license, you can go in the other part of the store that sells the good stuff. It's illegal to possess explosive and aerial fireworks in Delaware. They also have a waiver to sign that agrees you will use the fireworks legally.
I guess in Pennsylvania it's illegal to sell the good stuff to PA residents, but not to out of state visitors.
In one of the MD boarder counties (Montgomery maybe?) it at least used to (and may still) be illegal to posses even sparklers or fountains. And they were serious about enforcing it. Cops would stake out VA fireworks stands and radio in license plates to cops in-county. Since even possession is illegal, it's a confiscation and fine the moment you cross the boarder.
