Do users of What'sApp have the option of indicating whether or not they are users of the service? If yes, then I assume your friend doesn't want it known or quite possibly has your blocked. Hopefully, members who use that service will reply soon.
You can see her in your favorites?? (that's the phone favorites? or whatsapp favorites?)
I'm assuming whatsapp favs but if so, it doesn't make sense that it says she is not a user (it woulndn't show her in favorites).
Remember that you have to add the country code with the phone number. +xx-xxx-xxxx
Anyways, double check her phone number or tell her to write to you first.