There are genetic, dietary and fitness components to obesity. This is not disputed. At least one of these components is outside of the control of the individual. Others can be influenced.
If someone is chronically obese, the best thing for them is to work on what they can control with professionals, which might normally include a dietitian or a personal trainer but they might also benefit from work with professionals in other fields -- psychiatry, comorbid diseases etc.
And having friends support them in this process helps too. (I don't think their friends should tell them "fat is beautiful," by the way, but I'm not a professional in any of the relevant fields, so does my opinion even matter?)
At no point do I think shaming the obese (which is basically what you're doing when you say "this is simple and it's your fault" does more harm than good.
Shame isn't listed anywhere as a treatment for any disease or disorder as far as I know.