by

 

If a person has lost a lot of weight, chances are they are proud of the weight loss. However, weight loss may be a result of a recent sickness. Since you don't know why or how the person lost their weight, it is safe to make a comment like "Wow, do you ever look great!". Don't refer to their weight loss, simply make a generic comment about their improved looks. If the person feels proud about their new body (especially if they underwent a diet and exercise regiment), they will tell you how they lost their weight.