I'm going to tell my friend that she's wrong to accept praise for her weight loss
Why do you think your friend shouldn't accept praise for her weight loss?
I don't think she deserves it, it's not that big of a deal.
Losing weight is quite and accomplishment and your friend deserves praise for that. You shouldn't be negative towards her for receiving praise. You should be happy for your friend.
I'm not happy for her.
Then it sounds like you have some other issues with her. It's perfectly normally for others to compliment someone who has lost weight. Perhaps you are jealous.