A person's body weight has nothing to do with their ancestors. Getting to 375 pounds means the person was eating too many calories and not moving enough.
Fat is active metabolic tissue. It doesn't just sit in the body and do nothing, it contains hormones and active cellular functions and acts as an "organ" in the body. By the time somebody weighs 375 (and even before that), by eating they are also feeding their fat. Their body mass is such that they will naturally eat to sustain it, which means they will take in the amount of calories needed to maintain 375 pounds. That is one reason it's so hard for obese people to lose weight - they eat to their body size. It's almost unconscious or non-volitional, and certainly the high SOS foods (sugar-oil-salt) in the food supply don't make it easy since foods containing these ingredients, as well as animal products, are addictive.
So they have to break the addiction AND eat fewer calories. Fasting is a good way to address the addiction side of things. Your taste buds pretty much "shut down" and are only reawakened when you are ready to eat again, at which point you can start "fresh" with a whole foods plant based diet FOR LIFE to avoid the addictive foods in the food supply (animals, fats, oils, sugar, salt).
I think fasting is better for quick weight loss than veggie juices. Any juices enter the bloodstream too quickly. You're also asking somebody who may not have exercised in a long time to do "regular" exercise on a juice fast. I think that's an overreach, TBH.
Fasting will motivate you to keep going because the losses even at 1# a day, are enough to see scale differences, which keeps people going.
I say fast for 30-40 days, take a week off to eat a healthy plant-based diet (following post fast feeding protocols), then if you want to keep fasting, pick it up again. Rinse and repeat.
My take on exercise: If you're not used to exercising regularly, a Water Fast or even a veggie juice fast are not the time to start. You have to stay motivated on one action and it's hard enough to fast without having the pressure to also exercise. If you want to walk around the block, do it. If you want to walk up and down your (or some) stairs a few times, do it. But don't feel like you have to.
Focus on one action. When you're ready, start moving. The author of the Zen Habits blog has some great posts on starting exercise. Do LESS than you think you can do. If you decide you're going to walk around the block today but even that seems formidable, step outside your door and walk 20 yards up the street. Then turn around and come back. Like that. And see where it takes you.
There are also tons of YouTube and other streaming workout videos you can do if you feel like staying indoors and moving around. Go to YouTube and search for beginner aerobics, beginner cardio, beginner toning, walking cardio, or whatever. There is something for everybody on YouTube for fitness and you can burn calories without leaving your house. There are benefits to walking outside that you can't get from YouTube, but everything helps.
There is a woman who posts on the Video Fitness website forum who lost almost 100 pounds and all she did was change her diet and do Leslie Sansone walking videos.
And you don't need a ton of room or equipment to do video workouts. Jessica Smith has her own YouTube channel and streaming site and she has an amazing array of workouts, and in many of them, she's working out in a corner of a room in the space of about a 4 foot circle. You don't need a huge space to get your heart rate up, you just need the motivation to do it!