Regardless of why they are giving the ferrets away, I highly doubt they would be fine with them being fed to a giant snake. People give free pets away all the time so I don't see that as a red flag at all. My first dog was a 6 month old golden retriever that was given up to a shelter by her previous owners. The owners didn't get any money of course and there were no behavioral or medical issues with the dog. They kept in contact with my family for the first few months to check up on her. Some people just bite off more than they can chew when it comes to pets and like I said- that doesn't mean that they are okay with their pet being used as a feeder.
Bottom line is: He should be completely clear with his intentions. The owners have the right to know what's going to happen to their former pets.