What To Avoid Saying To Someone Who Is Losing Weight
Do Not Compare

People tend to say it without thinking, but the words "you already look so much better than before", often escape. Saying that they look better than before may do more harm than good. The chances are that the person losing weight is already doing so inside their own mind, so being aware that others are doing the same will most likely make them feel insecure. If this person wants someone else's opinion, they will probably ask, therefore given unrequested opinions is not only unhelpful but it is also rude. If it is thought that they look better than before, it is much better to give a general compliment regarding their outfit or disposition, keeping the risky subjects out of it.
Do Not Discourage

Most people struggle with diets and weight loss. A significant change in someone's lifestyle can be hard to keep up with, so it does not come as a surprise when people give it up. Losing weight is a huge lifestyle change; it often involves a routine of exercises along with a balanced diet. It can be a lot to get used to, so when someone says to a person who is losing weight that they will gain it all back, they are discouraged to keep going. Perhaps they will gain the weight back, and maybe it will happen a few times until they are able to stick to a routine to maintain their weight, but considering how hard it may already be, saying things that make it more difficult is the least helpful thing that someone could do.