The simple answer is no, french fries aren't a healthy treat for felines. (They're not really healthy for humans either, but they do taste good.)
For starters, french fries contain high levels of salt and fat, which can lead to obesity in cats. Obesity exposes your cat to various health issues such as diabetes, heart disease and joint problems.
Moreover, french fries are generally loaded with salt (which is unhealthy for cats) or other seasoning powders like garlic or onion powder (both of which are toxic for cats). Plus, giving your cat a salty food like french fries can cause excessive thirst and dehydration.
On top of all this, fast food restaurants sometimes prepare their french fries using oil that may be rancid, or that may have been used multiple times before being changed. This means that the oil may contain harmful free radicals and other toxins that could pose a risk to your cat's digestive system.
As always, consult with your veterinarian before introducing a new food item into your cat's diet! If your cat eats french fries and is showing signs of illness such as lethargy, diarrhea, or vomiting, reach out to your veterinarian immediately. If your dog requires veterinary care as a result of eating any unsafe foods, pet insurance can help cover the cost of treatment.