How to keep cooked French fries (for French fry dogs) warm and crispy?
Depends on what kind of fries you are using. If they're frozen fries— who knows. They're not all processed the same way— some have flour or batter coatings— aren't all made from the same material— some are made from formed, pureed potato and/or reconstituted potato flour— or cooked to the same level of doneness. If they're fresh potatoes which have been par cooked, they'll last in the refrigerator at least overnight before they dry out.

The other problem is your batter. If you're using a batter with baking powder, carbonation, or anything else like that to give it a little puff, it will flatten out if you hold it for more than a few minutes.

Generally, foods with batters are much better when you cook them immediately. If you only need to store it for a short amount of time, maybe less than half an hour, cook it then put it in an oven preheated to the lowest possible temperature (ideally there's a warming setting) on a sheet pan with a wire rack on it. It should keep the fries crispy, but the longer you keep it in there, the drier it will get.

Good luck!