Each Passenger should get credits for their flight if the tickets were purchased (not A+ Rewards Flights). In order to get credit both passengers A+ rewards numbers need to be listed on the itinerary. If your girlfriend did not sign up for A+ rewards then she will not get credited. However she can still sign up and get credits for past flights, up to a point.
From the Airtran website:
I recently joined A+ Rewards. Can I get A+ credits for flights I have already taken?
We will give you credit for flights flown within the last 90 days. Log in to your A+ Rewards account, click on My Rewards, then click on Request Credits. Enter the necessary information and submit your request. Verification of requests may take up to 72 hours for completion.