How do charcoal/carbon tablets help in better metabolism?

They don't help: Such tablets don't change a person's metabolic rate, nor affect her body's functions in any noticeable beneficial way. On the other hand, if she spends money buying the tablets, then she has fewer dollars to spend on yummy, sugary snacks... so buying the tablets might improve her health that way, by decreasing sugar intake... but that's pretty far-fetched.