Did some looking around, which you have probably done also, and noted that there was no definitive answer. An unripe eggplant is going to be hard and have more solarine which apparently some people react to.
However, since eggplant is a nightshade and some other nightshades ripen on the counter (think tomatoes) I would try leaving it out for a few days. The frost might have killed that possibility but maybe not.
Worst case, I would peel it, cube it and add it into a tomato based soup...say a minestrone or other vegetable soup, where it can either break down or, if it doesn't, provide a little texture without being the lead ingredient. If you use it in eggplant parmesan and it is tough, the dish will be inedible. If you use it in a vegetable stew, chewiness won't kill the whole dish.
Or you could cut it in half, rub olive oil on it and roast it in the oven. If it softens, use it in baba ganoush or in ratatouille. If it doesn't, feed it to the chickens.