I think the primary goal of 3FC is to support healthy weight loss with strong emphasis on the healthy.
I've actually seen it happen several times where someone will post something that doesn't sound at all good for them and other members quickly jump in and gently (or occasionally not so gently - depending on the tone of the OP
) try to steer them in a more healthy direction.
We're all here to get support and we all try to give an equal amount of support in return. If you are genuinely concerned for someone, and make sure the tone of your reply reflects that concern, you certainly have the right to let them know. If they joined 3FC, they probably have a least a little suspicion that something isn't quite right with the way they've been doing things anyway. Just be careful not to sound too judgemental - maybe start your sentences with something like "From everything I've read" or even "when I was doing that, this is what happened". I know I wouldn't want to ignore a possible cry for help, but I wouldn't want to alienate someone I might be able to help with a little patience either.