My friend's self esteem is so low - how do i help her with this?

Cheer her up. : You can encourage her, give her support and do things with her that she enjoys. Make her laugh and be there for her as a friend. She has to believe that she is a worthy, valuable human being in order for her to feel happier. Some people may suffer from such profound low self-esteem and depression that they may need professional help. You could encourage her to seek help if needed.

Suggest therapy: Probably the best thing is to suggest that your friend see a good therapist who will learn about who she is and what her background is and what her problems are and then will be in a position to begin to work with her to help her feel more comfortable about herself and to feel more confident about herself. Helping your friend make that step would be a great thing to do for her.