For one who has done yoga many times, yoga just isn't about the positions but rather finding your center. If you have knee problems you can adjust your positions to make it easier.
Some questions I have for you is how long have you and your gf been together. Is she set on going to this particular one or yoga in general? And what are you concerns?
How about this. Go for at least one class and let the guru know of your knee injury and see what he recommends. Tell him you want to try it out. If you open your mind to it you might find that you have nothing to worry about.