It's possible to amputate parts like that but it is very inhumane and you can screw up their urinary tract system because all that stuff is hooked up in the same area. Basically what a vet does is give the cat a vasectomy, whereas the othe method would completely screw up the urethr and vas deferens and the male parts in general. Plus as I said it's not very nice. It's kinda painful. You are restricting blood supply to that area which causes pain and eventually numbness, and then necropsy of the tissue beyond the rubber band and then it will fall off. You have to worry about infection, and then there's the idea that since that is a region that there is a lot of blood supply it may screw up and hurt the cat with internal bleeding. I do know they remove lamb's tails with a rubber band that way because they are too cheap to dock them at a vet, but I would never advocate it. I mean ask any man if he'd rather have a vasectomy under anesthesia or have someone tie a rubber band around his "balls". I'm sure they'd agree with me. So basically, it's possible to do that to an animal and it lives, but it's not a good method and it can possibly cause harm. It's possible some animals lived and some died and your friend's just didn't know it.