I don't know if your girlfriend just doesn't ride bikes or not but perhaps you should try to sell her on the positives. Here's the reasons my lady likes it better than riding solo:

* She doesn't have to pedal very hard
* Wind protection
* Doesn't fall behind (even though I'd wait for her)
* No decisions about when to cross roads or which route to take etc
* It's easy to talk amongst ourselves
* Lately she's started using the time to talk on her cell phone

I think it's important to let your stoker know what is happening at all times (for their comfort). "Turning left", "going up a gear", "pothole", "speedhump" etc. Also important they have a comfy saddle as it is a little bumpier back there.

But then again, plenty of people who talk to me about their (usually) only tandem experience describe it as "frightening", "a hellish experience", "death ride" etc. I've never found it anything other than a total blast, and very easy. So I guess everyone's different.