If it's on the activation screen then she should see part of the email address (first letter of the address and the email provider) on that screen. Or she could try logging into each of her accounts on http://www.icloud.com and see which one it shows on in the Find My iPhone section.
If she still can't find it, and if she was the original purchaser of the iPad (i.e. it wasn’t bought used) then she could try going to an Apple Store (she should make a Genius Bar appointment if she doed) or try contacting iTunes Support online (http://www.apple.com/support/itunes/contact/) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it