You'll need to go into the settings and add her account, then make that account the primary account on the phone, then delete your account. Here is a link (http://www.samsung.com/us/support/owners/product/SGH-I997ZKAATT
) to a source of the owner's manual for your phone.
You want to go to settings/ accounts and sync/ manage accounts to set up her account. You may have already done so. However, best perhaps to wipe the phone clean before setting up her account. This can be done from the settings/ privacy/ factory data reset area, which will keep the google servers from using the account backups to restore your old account on the phone. However you will lose all installed applications by doing so. (See pages 120-121 of the gingerbread version of the manual.)
If you choose not to do a factory reset, follow the instructions for configuring your phone (http://downloadcenter.samsung.com/content/UM/201202/20120207034812540/ATT_i997_English_User_Manual_KL2_F3_Gingerbread.pdf
to add her account, then restart the phone, delete your account, restart again and sync her account and see if the phone won't maintain her settings.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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