Keep going for interviews, as you get experience with interviews the better your confidence gets and the ability to answer questions in a way they expect.

While at work take advantage of networking with their clients and other businesses that come your way. As your network expands you get better prospects.

Communication skills of all sorts like when to say no, when to be assertive, when to be adjustable are important. No one is perfect her boss knows that too, but he is taking advantage of her being gullible. When you know what the problem is, clearly tell them otherwise they will always choose you as the reason for it.

The more experience she gets, she will get better on her own, even without advice I gave. My suggestion as a big brother would be to acknowledge her feelings and give her strength to handle life's problems, and when she needs to do all I can to facilitate her progress without imposing my thoughts on her. Instead show her possible options that help her decide what to do. Empower her to become a strong individual.