You have several choices but fixing a 15 year old electronic collar isn't going to be one of them. There is an outfit called the Collar Clinic I think, or something like that. You can find out with at Google search. They sell new and refurbished dog collars so you can get them at a good price. But an electronic collar is just a training device, meaning that the owner has to "train" the dog. If the owner isn't up to doing that then the collar is a waste of money. Just getting the collar is no guarantee of anything. I just bought a Tritronics unit in September and am very happy with the way it has allowed me to correct some problems I had with my dog. If you are willing to spend the time training the dog the collar is a good solution. If you can't do it yourself then you need to go to some different alternatives.
There was a dog expert on the Oprah Winfrey show a while back I think they called him the dog whisperer and he's in Southern California some place. He is a real wizard with dogs. I'm sure he could help but I doubt he'd be available. But the point is there are plenty of other people that can help with dog training. Any decent dog trainer can break a dog of aggressive behavior. It is rare indeed to find a dog that can't be trained. But if your friend had such a dog even then putting it down is not a good solution. Usually a dog can be relocated to another home and/or owner that can deal with the problem. Better to find a different home for the dog than to have it killed.
Aggression in dogs is a common problem, finding a dog trainer to correct this problem should not be too difficult or too expensive. Between the electronic collar as a training device and consultation with a professional trainer, breaking a dog from overly aggressive behavior isn't a big deal. But I'm not surprised that it is a woman that has the problem. Most women don't have the ability to command respect from many dogs. Get the dog and the owner some training and then she will most likely be able to handle it.
Hawke