Mom's best guess is that this is an Osage Orange. They grow in the Midwest and other places, and are used as hedges, etc. They have a fairly large spiny looking fruit that is not usually eaten.
She says that a true key lime only grows in the most southerly regions, and since she was born and raised in Florida, she has seen a few. The one in their backyard never grew taller than 8 feet, but it also DID NOT spread. It had ping pong ball size fruits that turned light greenish-yellow when ripe. They also smell like lime. It is quite sensitive to frost and freezing weather, where the Osage orange would not be. The other possibility is sour orange......
Little Bo Peep