That is absolutely terrible advice. Ferrets are very strict carnivores. Unfortunately, exotic medicine is a specialty field and many vets who "see" ferrets are also not terribly knowledgeable about them.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry