I think she needs to say something plausibly bad enough to quit without another job over, but that doesn't reflect negatively on her or her prior employer. This can be anything from the changing amount of overtime required to taking care of an ill relative (if true). Without any reason or a weak one, it can raise all sorts of speculation, was she fired, how is she flakey, none of which are desirable in a new hire.
If you are interviewing in response to a head hunter call, then you can say, I wasn't looking for a job, but when I heard about this opportunity, it was so interesting I wanted to learn more.
If you have a job and are looking, you can say all sorts of things around looking for new challenges, limited in your current role, heard great things about the company you are interviewing with and so on.
If you've quit, it is, at least in my field, sufficiently unusual that it would be good to give some reason to head off any negative speculation.