I know someone who wants to use hcg human chorionic gonadotropin diet. is it safe?

No.: And little evidence that it works. And it is not safe.

500 cal/day ??: The weight loss is due to the severe caloric restriction. In men with low testosterone and normal testicles, it may work better because the HCG will increase the testosterone level. In women, HCG may increase progesterone. Higher Progesterone may lead to less anxiety and therefore less eating. Overall, smart eating plus increased activity is a better choice.

See below: This was a diet that was promoted in the 1950's and then someone "rediscoverered" it and it has become just the latest diet fad. It did not work then and it does not work now. It restricts calories to 500 per day which is basically a starvation diet. This will cause weight loss initially but as soon as you go off of it the weight will rapidly come back due the suppressed metabolism.