As far as I know, cycling would be a low-impact cardio option. It would require buying a bike if he doesn't have one, of course, which for some isn't practical. It's the method I use since I have one, and have had a lot of success with losing weight and getting healthier without putting strain on my lower back and hips.
Another option is water-based workouts. They're usually more popular among people with joint trouble - such as those with arthritis - and people who want a more intensive workout from the water resistance. There are usually community classes offered, and I'm sure there are online videos too. I found a link to some low-impact exercises he may benefit from, which I found under the Google search "Low joint impact exercises":
Low Impact.
I also agree with Rainbow.sprinkles is that a lot of weight loss comes from diet changes. If he has an unhealthy diet that he hasn't shifted yet, it would be worthwhile to look into it. Diet changes can also help heal old injuries.
Additionally, he may benefit to look at ways to help his joints. Visualizations are good, and can benefit weight loss as well. There are a few around, you can find a few under the Google search "Weight loss guided meditations".
Regarding visualizations for the joints, that will take some practice. A good one that is simple is to imagine river water flowing into the joint, softening and carrying away the leftover tension, stagnant energy, and scar tissue from the old injury. Have him experiment with a few visuals if he's interested, and go with whatever he feels is right.