*TL/DR: Any sport (and swimming is an excellent one) will work, if done consistently, with applied hard effort and together with healthy diet and caloric deficit. No way to do it without clear goals and determination. To lose 80kg of mostly body fat, one would need 2 years focused effort, at least. It could be the one of the hardest yet most rewarding journeys one could take. Such that will not only improve the health but likely also self-esteem, general lifestyle, lifespan, work ethic and basically just about everything for this person.*

To burn 1kg of body fat, you need to get a deficit of 7000kcal (actually 1kg of pure fat has 9000kcal but the body stores water and other tissue to support the stored fat, so if you lose 1kg pure fat, you would have lost more body weight). If by including sports one burns on average 2500kcal a day and consumes 2000kcal a day (500kcal average daily deficit), it would take abound 2 weeks for a kg of body fat. But this can be a consistent long-term reduction. Good guideline to achieve sustainable fat loss, without yo-yo effect, is staying on deficit of about 20-25% of caloric consumption. 

This caloric deficit can be achieved by consuming less food (less calories in) and being more active (more calories out), preferably both at the same time. Eating healthy, calorie-sparse unprocessed foods makes it easier to manage appetite. Those foods are nutrient dense and fulfil your body needs unlike heavily processed thus named "junk" foods. And working out harder makes your body burn more energy and build/maintain muscle - the tissue you need to keep burning fat. Combination of high-effort activity and healthy higher-protein, low-sugar diet would make your body increase it's caloric expenditure, maybe such a huge person could burn 3000kcal a day, then the daily deficit could be 600-700kcal and lose 3kg a month instead of 2kg...

And actually swimming is much better than jogging for fat-burn - it burns more calories and allows to make more intense workouts (bursts or fast freestyle swimming) without straining joints or spine. The key to be taken is that any activity that helps burn substantial amounts of energy will do. But even if you work out a lot, it's very easy to out-eat the benefits of physical exercise. One hour of continuous swimming will burn 700-800kcal. One pizza is between 1000 and 2000kcal. Sport and diet (healthy, calorie-conscious) should be used together. 

And this is why weight loss is result of long-term habits. It has taken years of caloric surplus to get to gain substantial amounts of weight, it can't take months to reverse the process. This is slow (yet, very rewarding) process. There is no quick-fix, there is no diet or pill that can do this without hard effort and determination. It requires huge amounts of long-term motivation to stay focused on your goal. The goal of losing around 80kg of body weight is achievable within about 2 years of hard work. In my experience coaching people, the motivation to change yourself usually comes from break-ups, ultimatums, sometimes as attempt to get out of long term depression, new-found health problems or other sources of personal epiphany. But I have never seen it work if the person doesn't really want it and is doing it for someone else.

Edit: One book that I read and really helped me was "Burn the Fat, Feed the Muscle" by Tom Venuto. I really liked it because it emphasised not on specific foods or routines but how to set goals, measure progress and change your mentality towards a consistent healthy lifestyle. It thought me that there are no shortcuts but that you could look the way you dream, not the way you believe it's possible. Hard work and consistency is the only way but once you change your mentality it gets much easier. It also explained macronutrients and how to make smarter food choices.