Question:
Welcome to Basenotes, CarvalhoN!
From the little you´ve told us, it´s absolutely impossible to guide you in any direction, imho. Maybe you know some scents that she own, and/or like, to give us a clue??
Else, I think you should ask the SA:s at the perfume counter for advice, and then do some sniffing of what they recommend. If you find something you personally really like from that; -buy it!

Answer:
I also think this a very difficult undertaking. As we all know, one man's poison is anothers elixir. I agree with the suggestions to make an outing of testing different scents and letting her choose her favorite.