What exercises can an extremely large person do?
I'm not large, but I am 64, with a lot of injuries in my past (and a lot of joint pain) and I started with True Beginner. It's a great program, and Justin offers modifications to help you get started. Kudos to your friend on the decision to get healthy. The people in Justin's videos are real people who have limitations with age, weight and injuries, not elite, super-fit athletes, so it's much less intimidating.
As everyone else has suggested, if she's capable of doing something like True Beginner, that's where she should start. If even that is too much for her, it sounds like she needs to be doing something that's non-impact. and where she doesn't have to support her bodyweight.
If she can be convinced, it may be best for her to join a gym and start out on the recumbent bike. It's about as non-impact as you can get but it will still burn a fair amount of calories. She would want to combine this with a healthy, but calorie-restricted diet to lose weight. Once she has lost enough weight she should be able to do the sorts of programs you find here at DailyBurn.
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