Dog Shock Collar - 4500FT Dog Training Collar with Remote, IPX8 Waterproof Electric Dog Collar with 4 Training Modes, Security Lock, Rechargeable eCollar for 10-120lbs Small Medium Large Dogs

This is what I used: https://www.amazon.com/dp/B0FN7S5WMJ I would have liked something slightly more sophisticated, that also would automatically shock, just once, if the shock unit got too far from the remote. It would be a kind of invisible leash. But this model gave me almost everything I wanted. Fine control of the voltage, shock on demand with one button. A remote compact and simple enough that she can carry it in her purse and use it without looking, after a little practice.