You have a tough thing here. When negative thoughts take over the brain it can be hard to overpower it with good thoughts. Your friend needs to find something that she feels confident about and use that good energy to slowly build up her confidence. By telling her multiple times that she is awesome will not help her at all. People don't listen but if she can find something that she is good at and she knows that other people (not just you) know that she is good at it then she can start to appreciate herself.

Hope this helps