I can relate to your situation. I just got into BP's and I did not want to end up with an aggressive one. Long story short, I got a female het pied for a deal at a show and she was striking at me soon as I got her home. Every time I went to get her out of her cage, she would hiss and get in that S position. I found though that once she was in my hands, she was fine. If I had her out in my hands she would calm down and be cool. If I put her on the ground however and let her crawl around on her own and then try to pick her back up she would strike at me.

So that went on for the first two weeks or so. But I can say after about a month I noticed she calmed down some. She still hisses at me sometimes, but no more striking which is great.

So my advice is ask your girlfriend to give you time to work with the snake. I'd ask for a couple months at least to see if you can get your snake to calm down. If she doesn't calm down in the agreed upon time and your girlfriend still has fears about her you can rehome her then (snake, not girlfriend). But I think if you can get more time you may be able to have your cake and eat it too.