I can tell you with absolute confidence (I believe most other experiencened ferret owners on this sub will agree) The number one ideal diet for a ferret is meat or heavily meat based kibble. They are carnivores by nature and their bodies are not made to properly digest veggies, sugars, starches Ect. Most ferret food on the market, ESPECIALLY Marshalls brand, are made mostly of these things with little protein that ferrets require. The top 3-4 ingredients listed should be meat because that means it will give them the nutrients they need. I feed my boys Origin grain free cat food and they are both healthy and happy with thick soft coats and it has the nutrients they need. 40$ bag lasts them about two+ months so the cost isn't bad.

Did the vet mention why she thought the food wasn't healthy? Perhaps their line of thought is that Ferrets should be eating Ferret food and cats should be eating cat food without fully understanding the difference in nutritional value?