Coffee in a rattlesnake recipe? Why?
It's not a mistake. It's there because people like the taste of coffee in meat dishes. It adds some richness to the flavor, definitely something that works well with meat, and I doubt rattlesnake is any exception. I've had chili with coffee in it, and plenty of barbecue rubs with coffee. (I don't remember a specific recipe I've had, but for example these ribs look good.) You can find recipes for all sorts of other things, e.g. beef roast with coffee, pork tenderloin with coffee, or coffee beef jerky. You'll see dark chocolate or cocoa powder used in a lot of similar ways.

Unless you have a coffee allergy or can't get coffee, I'd try the recipe as written before you write it off. You could use instant if it's just that brewing real coffee is a pain. If you really can't use coffee, I'd try cocoa powder and water, in a ratio like hot chocolate, minus the sugar. You could also substitute milk or water or stock or any appealing liquid if you don't want to use coffee, I suppose. You'd still get a gravy, just not the same gravy, not the one the recipe author intended. 

And of course, I'd give the same answer about any meat recipe with coffee in it, not just this specific snake recipe. It might be surprising if you haven't seen it before, and I guess it might not be as common outside southern US cuisine, but it's a good idea.