NO Joke I do this all the time;
1).move the oven rack up to the highest level.
2).Turn on the top broiler in your oven.
3).spread the french fries in a single layer on
a cookie sheet and place on top rack in oven.
4).check them every 30 seconds to a minute.
5).as they just start to brown slightly remove the cookie sheet.
6).Carefuly and quickly turn the frys over once and return to the oven.
7).repete step 4 and then remove from oven & pore them on to a double layer of paper towels on the counter. Salt to taste, wrap them up in the paper towels to keep them warm and eat.
8).Don't forget to turn off the oven.
I've been doing this for years. never reheat them more than once they turn into little bricks.