While it's been about 11 years since I interviewed, here's what I remember. The format may be different now.
Usually the first interview is a group interview. Typical questions. TMAAT you had a difficult situation with a customer, co-worker, etc? How did you resolve it? What are your strengths, weaknesses? You are always being watched. From the minute you walk in the door, to when you leave. They look at how you interact with the other candidates and the interviewers.
Two weeks later I was called back for one on one interviews. One person was from the people department, one was a line F/A and one was an inflight supervisor. Each one asked pretty basic interview questions, but mostly we just talked about random stuff. I was working for another airline at the time so they asked a lot of questions about that. The F/A and I had gone to the same university so we compared notes on which were the best bars to pick up chicks. Basically, they want to see if you are the kind of person they would want to be stuck with on a three day trip.
Business attire is a must. Just tell her to be herself. They do a good job of making you feel comfortable. Remember, this info is pretty old, and my memory of my interviews is pretty fuzzy. Maybe someone else on here will offer some more up to date info. Good luck.