Question
My granddaughter was hunting mushrooms and stuck her finger on a honeylocust thorn.  This was a month ago.  Now her finger is still swollen, will not bend fully, and hurts at times.

Answer
You can rule out toxicology here, as Honey Locusts show no signs of toxicity, and only the seeds of the Black Locust are dangerous.
So she may still have a piece of the thorn still in her finger, and it's acting as an irritant. There is also the chance of an infection.

If it weren't near a joint I'd say treat the infection, and see if it dissolves or expresses out, as splinters are wont to do. But near a joint, there's the potential for damaging the joint.

Try soaking the finger in warm, soapy water with Epsom salts for about 20-30 minutes, not only to clean it but also to soften the skin. Then, with a sterilized needle, tweezers, alcohol (not for the wound -- to disinfect the equipment and the skin before you explore) and a magnifying glass, see if you can find the entry channel, and follow it down. Don't go overboard and hack a gaping crater in her finger -- the ideal here is to be extremely gentle and also persistent. If you can find and remove the splinter, hit it with hydrogen peroxide, bandage, and watch it heal.

If you can't find the thorn, or can't get all of it out, an X-ray and a man with a scalpel (and a license to practice medicine) is called for. If it is near a joint I would go ahead and see a doctor.