Any chance of taking it to a bike shop? They will hate you but they are more likely to be able to get it apart and figure out what you need. What you're running into, by the way, is exactly why they are referred to as Bike Shaped Objects. They are built to be cheap and gently or rarely ridden. Any kind of hard miles and they break and are not built for repair.

You may consider just getting a new wheel. If dad weighs less than 200 lb. you can probably just get a straight freehub replacement wheel for maybe $50 plus the cost of the new cassette.