When I lost around 100 pounds years ago, I did it on a mountain dew and rice cake diet (not recommended). That introduced calorie restriction, and it worked. After losing weight, it eventually motivated me to find and eventually adopt a diet that was long-term sustainable.
Hopefully the path my father takes is similar. Step #1 isn't always perfect, but it's much easier to take a single step than it is to try and run up a flight of stairs.
A former roommate of mine did the same. He went on an all fruit and vegetable diet, going from around 280 pounds down to about 170. That "beginning" diet wasn't sustainable, but it helped him lose weight through calorie restriction. Now he's a extremely athletic cyclist, and eats a very healthy well-balanced diet. Step #1 wasn't necessarily perfect, but it eventually got him on the right track.
A lot of the hard research on refined sugar appears to just be coming into light. It'll likely take several more years/decades before its effects are truly understood. Human nutrition in general is AMAZINGLY complex, as there are so many variables interacting with one another.
I find it's best to question anything anyone says on the internet regarding nutrition. Ensure they cite sources, state their educational background, etc... Even someone who's a Doctor doesn't always know a lot about nutrition. Typically your best sources are people with PHD's in nutrition, active in the field.
Almost everyone on the internet doesn't fall into that category. So when you end up with is a lot of "snake oil" advice mixed in with the good. It's a tough problem that needs a solution: "Verification of the accuracy of online information." It extends well beyond nutrition and fitness. The solver of that problem has the potential to be larger than even Google. :)