Awesome story and great girlfriend! I have a somewhat Similar story when I hooked up with my gf two and half years ago. On our first date I had Opeth Damnation playing in my car stereo at a very low volume just for some mellow background music. So the music topic came up and she said she is digging what was playing. I told her that was very different from what they usually play and left it at that. She certainly wasn't ready for BWP or Deliverance. So I learned she was the typical chick who digs Christina Aguilera, Evanesense, Sade etc. But she also had a liking for stuff I dig too like Tool, Old school Metallica, Soundgarden, AIC. So I knew she could stomach prog since those bands have prog influences.
So I figured she could dig DT so to the easiest way to introduce her to DT is by watching the Budokan DVD. She is also second generation Japanese American so why not play a concert that took place in Japan? She was blown away by the musicianship but disliked LaBrie because he sounded so flat during the whole thing (which I must agree).
Fast forward to today she now has a ring on her left hand and she thinks that ADTOE is their best album to date. When she first saw the video of OTBOA it became an instant favorite. She liked DT before, but now she is a hardcore fan.