The simple answer is: it depends, but the short answer is probably not in many cases--often baking is better way to reheat fried foods.
What kind of food, what kind of frying?
Frying depends on there being sufficient moisture in the item being fried that the water in the item evaporates on contact with the frying medium (usually oil) so that the the food is essentially surrounded by a barrier of vapor escaping from the item. This is what prevents the fat from entering the food and making it greasy.
If the food is dried out, frying is going to make it greasy.
Even if the food is not already dried, additional frying will continue to dry and crisp the item. Also, frying is generally a very fast and intense cooking method, so may lead to increased risk of overcooking or burning the food item.
In the case of French fried potatoes (which I am inferring you are meaning by "fries"), the classic cooking method often employed is a two stage process:
- Par-frying at a (relatively) low temperature to cook the potato through
- Finish frying at a higher temperature to make it crispy and hot for service
In this method, the finish fry is not just reheating the potato--it is a part of the cooking process which is planned for in the recipe.
So the technique may or may not apply to other fried foods. Often, the best way to reheat a fried food item--although it won't be as good as the fresh, hot from the fryer item--is to bake it in a medium to hot oven, which is slower, more controlled, and doesn't introduce additional oil to the food item.
Of course, all of that being said, there are some people who like to refry Popeyes fried chicken to make it even more crispy, so again: it depends.