When you say
she managed to nail every question with laser-precision
What does this mean? Did she
- Provide the correct answer to every question?
- Demonstrate a deep understanding of every question being asked?
Regardless of whether she received coaching from her dad or not, it's the second point that matters. Let's step back and look at some common questions when hiring someone:
- Is this person a good candidate?
- Is this the best performing candidate amongst the ones you've interviewed?
Is she a good candidate?
Any candidate that does demonstrate a deep understanding of the question being asked and provides the correct answer, is a very good candidate. Any candidate that does not demonstrate a deep understanding of the question but provides the correct answer, is most probably parroting the answers from the internet (or in this case maybe her dad).
If the candidate appears to not demonstrate an understanding of the problems, discard them. Plain and simple.
Is this the best performing candidate amongst the ones you've interviewed?
Now, let's say that this candidate did in fact demonstrate a good understanding beyond just giving the right answer. From your OP, it appears that she did so more than any other candidate you interviewed. If this is the case, then she is indeed the best performing candidate.
Great! So you should hire her! Or should you? It appears you are asking one more question here:
One candidate has a background that grants her a privilege in terms of the coaching she can receive. Should I still hire this person?
We all have our privileges. I personally can read much faster than 97% of the population. It's a technique I learnt from members of my family. It gives me a huge advantage when reading long technical documents. Is it unfair that I have this advantage? Yes.
But ask yourself this:
She has maybe used certain resources that are available to her. These are resources that she didn't cheat to acquire. She simply happened to be someone's daughter. Is is correct of you to penalise her for that?
Edit (replying to this comment):
There's a big difference between having a generic useful skill and having had the opportunity to prepare in depth for a very small pool of questions. With good coaching and playing through the interview a few times even a mediocre engineer should be able to pass every interview with flying colors.
A mediocre engineer that passes an interview with flying colors isn't mediocre by definition. They are well-prepared. They are in fact passing with flying colors. Stop thinking that being prepared is a bad thing. It is a good trait. In fact, it is a necessary trait for anyone aspiring to be excellent in their field of work.