She'll need to let her employer know of the mistake right away, and they will issue her a W-2C ("corrected" W-2) showing the proper SSN. You'll then enter the W-2C into TurboTax to replace the old W-2 (which should be deleted). The W-2C takes precedence.
Enter the W-2C in the W-2 section, just as if it were a regular W-2. Once you've entered it, you'll be shown a list of "uncommon situations" and you'll check the box next to "W-2C."
You can file an extension with the TurboTax Easy Extension feature by clicking on
the Federal Taxes tab,
then Other Tax Situations, then
scroll down to Other Tax Forms and
select File An Extension.
