Gloria makes some really good points. BTW, my release word is Freedom.
I have been trained by one of the finest trainers in Europe, whose specialty is the Hovawart breed. Large, smart dogs who can think and evaluate the situation. German shepherds are like that, too.
One of the things that will help is immediately correcting the dog when an undesirable action occurs. The key here is IMMEDIATE so that the dog recognizes that his behavior is unacceptable.
Carry a water bottle at all times. When the dog does something undesirable, splash the dog. I'm not talking about a misting, squirt bottle. I'm talking about a dish soap bottle or a bicycle water bottle and splash the dog in the head. It will not hurt the dog, but it gives immediate feedback that this behavior is unacceptable. It breaks the concentration on the object.
You must be one step ahead of the dog at all times. Watch for behavior cues. Is the dog looking at the cat, tense and poised for attack? Splash before the dog even moves. Redirect the focus.
Is the dog blocking you? Splash. Regain control.
My dog walked around with a wet head for weeks, but her behavior changed. Dog puberty is a horrible time and bad behavior can develop quickly.
You can splash indoors, as long as you are not around electronics. A little water isn't going to hurt the floor or carpet.
When the dog is outside, watch for clues. Tensing, raising a paw slightly, eye movements? Splash. Keep the dog on a short (6') leash. Where is the dog looking? Focused on cats? Splash.
What are his temptations? Be aware at all times. The water must come quickly and, from the dog's point of view, out of nowhere.
This doesn't hurt the dog, it's not designed to hurt like an electro collar. Splashing is humane. It is designed to break the concentration on the undesired object and redirect the dog's focus.
This is a first, but important step. There is so much for your cousin to learn and so little time.
I also agree with those who have said to find a rescue group. If the dog is reported or goes to a shelter, the dog will certainly not be trained. The poor dog, who did nothing more than his animal instinct dictates, will be doomed without proper training. The rescue group can help if all else fails.