I agree; the interview is really to see if the candidate has the qualifications, is a good fit, etc. In my experience, unless they were really desperate to fill a job, and absolutely needed to have someone ASAP, the start date doesn't even come up, especially in the initial interview.
She may have a second one, and maybe more, which takes time. Also, being that its summer, and people are on vacation, it may take more time.
The only piece of advice I can give, is she's offered the job, and accepts, and it may conflict with your cruise, that she tell them right away. Some companies maybe ok with an employee starting, and then taking time off, for something previously scheduled, and if it means holding off another week, some companies may be ok pushing the start date back.
Don't let her do what a CW did at my first job. Same type of thing; trip to celebrate her graduation, but after starting, she never said anything, until the friday before the week she was going to be out! they certainly would have let her go, but they weren't too pleased they found out at the last minute.