This is an extreme, but obviously there’s better and worse things. Maybe it’s pretty comparable in the reasonable middle, but let’s not kid ourselves and say anything can work if we are talking whole body health not just weight loss.
ETA: obviously I'm not advocating eating only sugar or twinkies. The point is that adherence is an crucial factor, more so than is often given credit.
With that being said, metabolic disease produces lots of skinny diabetics, and that will definitely shorten your life span.
The ADA training the insurance companies recommend is weird too. It says, for example, that it’s fine to have a slice of cake, just have half. I assume this is because too large a change will lead to negligible compliance. I was at first shocked, but then wondered what kind of baseline recommendation would actually be effective.
How long did she stay on it? I imagine serious metabolic complications will take some time to develop. Moreover, she'd be missing out on some essential minerals and vitamins so I suspect she was using supplements? In that case the diet is not really chocolate, but chocolate + supplements