If you're Gwyneth Paltrow, the answer is "of course." Ross Mathews, a comic on the Chelsea Lately show, told People that when he saw the actress, she pointed to his stomach and said, "'What's going on here? I love you. Get it together."
Since then, the former 220-pound performer has lost 40 pounds by exercising and joining Jenny Craig, so Gwyneth's honesty seems to have worked. I don't think I'd say something quite that direct, especially to someone I'm not that close to, but I do think I'd try to encourage a close friend to make better choices if I ever felt it was warranted, and that includes if they were unhealthily dropping pounds. What do you think — is it your responsibility as a friend to stage an "intervention" if you feel a friend's weight is reaching an unhealthy limit?