you know what – this sounds scarily like the Mist / Will scenario and that definately *wasnt* herding … are you sure you have the behaviour interpretation right ?
just a thought ….
i would be >:D >:D >:D >:D >:D >:D >:D if you were letting my dog do this as a client. if you think about it from your clients point of view – you are letting this dog practice very unwanted behaviour and probably creating or certainly re-inforcing problems for the owner 🙁
you need to manage the hunt ALOT more effectively and until you have the skill to do this, i would reccommend that you dont take your dog (and possibly not other clients dogs either – depending on situation) on walks with the collie.