Keep it on a lead no matter what your mate says. Once it's seen the ferrets going in and out a few times and hopefully a few bunnys hit nets then consider letting him off lead. Take my advice even if you think your dog is bomb proof with ferrets pal as even if pup is, one nip off a ferret that's worked up will end up with a dead ferret and a ruined terrier if this is what you intend on doing with the dog, enjoy your day and don't expect too much from dig first few times