I would worry that DL will cancel both reservations/tickets as duplicate bookings unless one ticket is written as something like Lastname, Extraseat. Similarly, you might not be able to earn miles if DL notices the duplicate tickets.

If the purchased FC were in girlfirend's name, you could have tried to reschedule the return part of the award ticket to a later date you can use for $150 if it's now within 72 hours.

Finally, I would be concerned that the FA will notice the duplicate male names and a female in one of the seats and ask questions or even call for the GA while the plane is at the gate. BTW, how do you plan to board the flight and have both of your tickets scanned? If both aren't scanned, the GA will take one of the seats as a no show and assign it to someone else on the FC waitlist for a gate upgrade.