Stationary Bike, Good for Cardio?

Yes. My parents are both in their mid 80's and they swim and bike regularly. They ride outdoors mostly, though. Stationary bikes are a decent substitute if you can keep at it.

Stationary bikes are good for cardio..... if they actually get used. Riding a stationary bike is about as exciting as watching paint dry and more painful. They mostly end up as clothes racks in someone's bedroom. Going for walks would be just as beneficial and much more interesting

Treadmill is offcourse higher nevertheless it difinitily can coronary heart your knees after longer term. I in my opinion use the elliptical they're less complicated on your knees and still beautiful strong.

How old are they ? Stationary bikes are so boring most people give it up after a few days. They would probably like real cycling a lot better if there are bike paths close by. I see a lot of retired people on bike paths riding for exercise and even touring.