As long as the fries you left out overnight do not have any mold on the fries then the fries would be safe to eat when left out overnight.
I've left fries out overnight before and then heated the fries back up the next morning and ate them and they've been okay.
It usually takes a few days for the fries to get mold on them so they would still be safe to eat the fries even when you just left them out overnight.
If possible it's best to refrigerate the fries but sometimes I forget myself and leave fries out overnight and still ate them the next day.
The fries become a little stale but they are still safe to eat.
Just check for mold and they should not have mold when left out just overnight.
But if you left the fries out for a few nights then I would throw them out.