1. Do you have a written guarantee? If not, then you are totally fine in saying, "sorry".
2. If there is a written guarantee and it is vague then there is a decision to make and with that a change in the guarantee for the future (chalk it up to experience -- painful ones often help us remember to be very thorough in the future -- talking from personal experience. *grin*)
Okay, now that that is out of the way. Saying no isn't always the best service oriented response though you may be in every right to do so.
" I described her in a completely opposite way."
This has happened to me. This is actually a correct read in a weird and twisted way.
Most querants that I have, let me off the hook because they realize I got the details right but attributed them to the wrong person. This is "okay" in my world and in the world of many of the people I read for because ultimately they are just looking for insight and ideas for how to proceed.
Here's some ideas to bring across to her: 1. You were explaining the "shadow self" -- everything she fears and everything she doesn't want to be; 2. You connected but the lines got crossed because what you do isn't "linear" like it is in the "real" world.
I would also ask her if she got "anything" out of the reading? Depending on how gracious she is, I would suggest either a partial refund (if you have a guarantee but it's vague) or another read given to her (if you have no guarantee) and view this as a "learning experience". In that instance, ask her what she really wants to get out of the reading -- what's her "end goal"?
Hope this helps in some small way at some point in time.