The Apple ID should be in the form of an email address.
If you go to the site http://iforgot.apple.com/ you will see an option in the lower, middle of the page labeled “Forgot your Apple ID?”. If you click this, it will ask for some rather simple information that you should know as the owner of the account ( first name, last name, current email address plus the ability to enter additional email addresses if it has changed since you created the Apple ID).
Once you have the ID, if you don't know the password ***** you can return to the site and reset it.
If I understand correctly, you determined which ID was hers on the computer. Did you have to reset the password?
Test out that it is working properly by signing into the Apple ID management site:
https://appleid.apple.com/ (under "Manage Your Apple account")
Once you have confirmed it is working, restart the iPhone and try try using it again.