Erica,
Err, join the club. There's not many of us who have breasts but it's amazing what can be achieved with a little tugging and squeezing. Why not buy some cheap forms off eBay, if you're a light frame I'd go for a B cup, if not a C. Give it a go. I'm sure your GF can sort out a suitable bra with you. If you don't try you'll never know. If the dress is a bit too racy, a bit too revealing a neckline then there's the opportunity to shop for something more suitable.
Having such a supportive GF is a gift. Embrace the opportunity or one day you'll be thinking, "I wish I'd....".