I read the original post as concern for a friend's health. Because being 50 lbs overweight does lead to all sorts of health problems. I also didn't see condescension in it, at least no more than anyone else has about other peoples' dietary habits, which can rank with religion and politics in terms of divisive subjects.

I don't get involved with friends' diets unless they specifically mention that they're trying to make a change, and even then I usually just try to be supportive in a general way, regardless of what I really think about it. If a friend or relative started talking about something specific like a detox diet I might ask what they hoped to gain from it, and then might indicate if I thought a different approach would be better. If it was someone I didn't know that well, I'd stick with a vague supportive comment.