French fries are high in fat and salt, which can upset a cat's stomach and lead to gastrointestinal issues or pancreatitis.
Consuming them regularly can also contribute to obesity and other serious health problems.
Suggestions
It's best not to feed your cat French fries.
If they accidentally eat one, monitor them closely for any signs of being unwell and consult your veterinarian if necessary.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.