The wierd thing is, like Ideation says, they used to live on it for years! Just shows how adaptable their systems must be cos you'd think they'd show signs of malnutrition or something.
When I first get a ferret when I was 7 I was told by the bloke I got it off to feed it bread and milk so that's what I did, and it 'seemed' ok, it probably wasn't in perfect health in hindsight, but it looked healthy and was full of energy....wierd!
They definitely 100% need a meat diet though, even if its shitty dried biscuits, ideally flesh but if that's not an option for someone for whatever reason