Some people struggle with weight. For others, they struggle with school; however, how does someone handle a friend that has no self-confidence whatsoever? As a close friend you can remind them repeatedly how their outfit looks amazing and their hair looks perfect, yet, your opinion will not satisfy how they feel. Even if you are telling the truth, dealing with a friend that cannot stand to look at themself in the mirror is not going to believe you when you shower them with compliments.
There is only so much you can say to a friend until that matter because mute. You can give a friend reassurance until you are blue in the face, but that is not going to change their opinion when they are stubborn. Instead of giving them compliments tell your friend areas of improvement. Offer help when you are able to. They are more likely to take advice then they are to take a compliment. Gradually ease into praises. When they do not except your compliments, call them out in a non-threatening way. Ask them why they do not perceive themselves as others do. Instead of noticing your friend as a martyr, see them as a friend in need. Helping your friend see themself in a better light is difficult; however, if you are able to make even the tiniest impact on their life, you can slowly change the way they feel.