YSK: Going on about how skinny/thin someone is, or focusing on their weight in a discussion isn't a compliment and in most cases is seen as an insult.

When it comes to speaking about anyone's weight it will make them feel uncomfortable, especially when in a group. Deprecating your own weight to compliment theirs will make this person feel uncomfortable in general.

If you want to compliment someone for how they look physically, instead tell them they "look great", "look healthy", or just simply highlight the fact

In the case you don't like the way someone looks physically - This person will most likely already know how it is and you're not being helpful. If this person is indeed making themselves sick, this is information on how to confront the person.