I'm not suggesting an easy answer, but someone who is already grossly obese
who continues to eat high fructose, etc. junk food at the rate you describe is
quite unlikely to be free of health problems resulting from this as they get older.
That has been my experience, at any rate.
Of course, we can never "predict" who will get away with it
and it is always tough to "measure" what might happen.
I face the same issues myself to a far lesser extent with my husband's health choices
and I'm having a hard think about what I want my "golden years" to look like.
Wishing you and he the best in any case.