Contrary to my first answer, but this point needs to be made somewhere:
Do you want to test the candidate for her ability to follow business culture?
Waiting rooms are for waiting, for, you know, a few minutes. It's considered to be a burden to the company, via the receptionist particularly, to entertain someone much longer.
Furthermore, whatever the candidate's relationship with her parent may be, in interviews we try to demonstrate full separation of personal life.
So in the other answers (including my other answer, honestly) you are witnessing:
- A bunch of answerers trying to say that it was probably not a problem to violate this business convention
- Some answerers trying to point out that something is wrong with what happened, but falling for the red herring of considering the relationship between the candidate and her parent.
What is going on here is that it is wrong, as a matter of business culture, to use the waiting room area this way.
What the answers are generally urging you to avoid is holding this against the candidate via psychoanalysis of the candidate's backstory. This is a pure self-fulfilling prophecy: it's a problem because it's fishy, because the interviewer may find it fishy...
In my honest opinion it's far more likely the candidate is just probably young in her business career and has a totally normal relationship with her parent where this was the more convenient thing to do. You have a chance of making a solid hire for treating her that way.
But you have the option of penalizing the candidate for poor understanding of business practices. Simultaneously you should learn how to react when someone commits a poor business practice. It's your judgment call how this impacts her ability to do the job. Most answerers here are more of the culture of moving away from vestigial business traditions. In particular most answerers here would love to see a "common sense" approach to the waiting room take hold, namely that it is okay for someone's parent to wait there.