I think it depends on the reason someone loses the weight in the first place. Some do it for health concerns, some for cosmetic reasons, some due to insecurities, some for an event, some for longevity, some because its not in their control....hence keeping your mouth shut is better. And my favorite kinder way to approach this is asking someone "how have you been? there is something noticibly different about you?" which leads them to share why their physical appearance has changed. And if they dont say anything about it...you leave it at that. We need to learn to allow people to simply be without judgment.