You are in the wrong only if you didn't tell the guest you would be unavailable, @Electra5 .
What I do if I have a guest staying, and I have to be away with my phone turned off is this: I tell them that will be the case, I give them a backup number to call, and I make sure the backup person will indeed be available for them.
Not sure what to do to remedy this now. If you did all that, you could remind her of it. It sounds like you have reminded her that the information she needed was in the manual. That's okay, but a guest usually doesn't say, "Oh dear. Totally my fault then!"
You could apologize. Is she still there? Bring her a small gift maybe. I do think that the worst that will happen if she "reports" you is that customer service will tell you to be available to all future guests and you will say yes. If by "report" she means she will mention it in the review, then fine. Not the end of the world. You can reply that you have taken steps to ensure it will not happen again.
Hope that helps a little.
