Is the user logging in through Facebook? There shouldn't be any reason for that I can think of. When you say that she is trying to connect her account to "her facebook page", does she mean a real "Facebook Page" or just a personal Facebook profile, like 99% of user's have? Facebook Pages are something else and not something you can usually log into.
If that is what she's trying and getting that message though, can you tell me where the error is being reported? Is it in the login popup window for Facebook or on your site itself? That will help us narrow down possible causes.
Also, are you having any issues with other users logging in using Facebook?
Thanks,
Alex