Hi mate! I haven't post anything for a long time but this topic is specially touching for me, so I thought I could add my two cents. First, I don't know how your girlfriend is, but if she has got good marks and tons of research experience, she is probably the kind of girl who likes to get things by herself. I say this because anything that you do, you better let her know first, there is no need to make her believe that she got into her dream school just because they didn't want you to transfer.
Once you agree with your girlfriend, then it is time to put your plan into practice. To my surprise, physics departments are extremely understanding when you explain this kind of problems. I say this because this is not the case in my country (universities don't control the funding, they can't help you at all), and because I had a similar problem and they helped me a lot. However, I don't think you should be completely straight about your intentions for the following reasons:
1) If you are not tactful enough, it will sound like if your are blackmailing them.
2) As I said, your girlfriend will never know what was the reason to be accepted in this school.
3) Even if you finally get what you want, this will not improve your image in the department. It might not hurt it, but if I knew that you were completely sure to transfer for this reason, I wouldn't think that you were the most commited student. Maybe, someone would love to see how young physicist love each other, but I wouldn't bet for that.
What I would do is to ask for help. Express your concerns, say something like "My girlfriend is applying this year, and god knows how my life will improve if she could only study here. Would you mind to give her advice and introduce her to some faculty to help her make a stronger application?" She will still get help, but this is completely legal (we have seen people doing really weird things to improve their chances, using the help of someone inside the department is not the worse by far).
I will also propose to ask for help from other departments. You didn't say where UX was, but almost every university in your country has a decent second option nearby. It might be better to travel a bit every now and then (obviously east coast-west coast won't work, but princeton-columbia or Boston might) rather than transfering and screwing up your relationship with your current institution.
I'm not an expert at all, maybe your current institution will understand your situation and will allow you to transfer without problems, but there is no need to force them to accept your girlfriend and neither to force them to accept your transfer petition if they are not willing to do so.
My point, ask for help and wait for the lottery of the admissions process to end. She is accepted, no one gets hurt. She is not accepted, you ask for a transfer.
I hope it helps, and sorry for my English, I'll try to fix my mistakes as soon as I get to Boston. Good luck!