It would be ok to keep him in the garage if it's well ventilated. My bf's family lives in Georgia and they have an indoor/outdoor cat that likes to hang out in the garage in the summer. They have to use a fan to get the air moving in there so it's not too hot, and the fumes don't get to her. There are ways to keep bunnies cool in the summer, but in a garage you have to be careful because a lot of things can be stored in there that give off bad fumes, like cars, paint, household chemicals, etc.
Also, do you have a puppy pen from your dog? That might make a good rabbit run, provided you're there to watch her in it. Like Jen said, bunnies need a lot of exercise and time on the grass is great for that. If they're outside, though, the run has to have a lid on top so that birds don't come at them, and you have to take it in before dark so that predators don't bother the bunny.
The best situation for the bunny is to have a cage in a well-ventillated area, plus a few hours (at least 1 every day) outside the cage, running. When it's raining, you might have to set up the pen inside or in a clean area of the garage.
Glad your dad is doing better!
