It is difficult to encourage or ‘help’ anyone who has low self-esteem because the very fact that they do not believe in themselves, and think they are unworthy, will make them find it hard to accept anything positive that you might say. Self-esteem grows out of self-love, which starts from how we were treated in childhood, whether we were appreciated or not, or shown real value for just being ourselves. Low self-esteem begins internally because it is rooted in a lack of self-acceptance, a feeling of inadequacy, and being worthless.
Thus low self-esteem is a rejection of the self, a loathing for who we are, and when we don’t like ourselves, it is difficult for us to appreciate that others actually like us, perhaps thinking they are being insincere, and would always be questioning that love. In fact, it becomes hard work after a time trying to convince someone of low esteem how wonderful or amazing they are, because they can only see their perceived ‘faults’, and have no self-belief in their talents or goodness, hence why your friend has to help herself, because whatever you do is likely to be doubted or rejected.
People who have low self-esteem tend to be perfectionists who seldom measure up to their own high expectations in their own eyes. They are constantly comparing themselves to others, constantly beating themselves up for any little perceived error or fault, believing that everyone else is better than they are. In short, their life is dominated by FEAR, which is fed and maintained by the negative experiences they might have had, and a marked absence of trust, which fuel their suspicions, especially a fear of never being good enough. Based on an unrealistic assumption of perfection in others, this fear pushes them to place themselves below others, always comparing themselves in a futile way which makes them seem even more unworthy in their own eyes.
Only the low esteem person can change their situation. What they need is an acceptance of their own fallibility; to give themselves some slack; to stop the impossible expectations, stop beating themselves up for any mistakes, and stop finding constant fault with themselves. All you can do in the meantime is to reinforce any positive thing they say or do, while deliberately ignoring the negative ones.
For example, encourage her to be more positive about her life, especially giving gratitude for the people and blessings she has. Pay her regular compliments, and keep affirming her as a valued and worthwhile friend. But make sure that when you react to anything that is said, you react mainly to positive, enhancing things, and remain deliberately silent on any negative action that maintains the status quo. In that way, she should hopefully welcome the positive attention you give, which should make her want to act in ways that get her even more of it.
But helping people of low self esteem is no easy task, because the responsibility for their feeling and actions lies entirely with them, not with anyone else.