To answer this question right off the bat no you should not be feeding fries or french fries to your ferret because overall it is bad for their health.
More Detailed Answer
As you can imagine fries have a lot of salt and additives on them and are deep fried making this food pretty bad for your ferret.
With all the oil and other unhealthy ingredients on fries it is best to avoid trying to feed this food to your ferret.
It really isn’t even good for humans and definitely not good for ferrets to eat.
What happens if your ferret eats fries by accident or steals them?
Ideally at this point you should closely watch your ferret to make sure they do not experience any negative health side effects.
If you notice them acting weird it may be best to bring your ferret to the vet for a checkup to make sure they are okay.
Most likely ferrets may experience some diarrhea and then be fine but besides that small amounts shouldn’t hurt them.
Another option is to give your ferret a ferret safe laxative but usually that isn’t necessary.
As mentioned above if you are really worried about your ferrets health take them to the vet or consult a local pet shop with more experience.
And no never try and feed fries to your ferret.