Ignoring all the sexists and stupid "joke" bits of advice this is the best thing you can do. Girls do not "see" much progress due to their naturally decreased test levels, this a great intro article ( http://www.defrancostraining.com/articles/archive/articles_female-train.htm ).
I am going to come out and make an assumption that your are training for mass(non-function hypertrophy) so every workout you are going for a "pump". If you girlfriend is doing the same exercises as you she will not progress, as stated above, as fast as you in the looks department. This can lead to, in the circumstance of ignorance as to why progress is slow to, lack of confidence, anger, sadness which is probably manifesting in this "boredom" you are talking about.
Give her a program that will "empower" her like a big compound movement program, I am thinking along the lines of Rippetoe/Madcow where progress for newbies is swift even for girls and you will be adding weight every week if not every workout even if it is like 5lbs. Then as the poster I quoted stated encourage her every step of the way and show her how much she's improved. It will make life so much easier if she is not "competing" with you.
As an aside I would also recommend going in the Powerful Women forum and checking out figureathelete. Also possibly get your girlfriend to start posting there so she can find people she can relate to.