I have been in the same situation as your gf for the past 2 years. I am a Canadian citizen with an American bf. I was also getting interrogated at customs, understandable, as they were trying to determine immigrant intent.
As a result, I filed for a Nexus card to mitigate this problem and it was virtually a painless process. Filled out the application, heard back within a few weeks, was conditionally approved, went in for an interview at the Nexus office at the airport, which was far less intrusive than I had anticipated. Met with US officer first, who just verified the info on my application (spelling of my name etc.) asked whether the card would be used primarily for business or pleasure - I simply answered pleasure. I did NOT volunteer any additional information regarding US bf, nor was I asked. US officer took my photo and fingerprints. Then met with CDN officer next, who basically entered my passport info, explained the program, do's/don't etc., took my retinal scans, showed me how to use the machine and asked me if I had any questions.
Your gf has nothing to worry about, she will likely be approved if she has no criminal convictions or anything else to raise a red flag with the US/CDN governments and also provided that she does not disclose any information regarding you and your relationship. As I said, I wasn't asked, so it shouldn't be a problem. Getting the card was the best thing I've ever done. My border crossings are now painless, even when I do have to briefly speak with an officer when entering the US, s/he simply asks if I'm going for business or pleasure, and nothing further, they're actually very friendly and always thank me for using the Nexus program. I haven't had an interrogation or any problems since.
I hope my experience and insights are helpful! Best of luck to you both!!
-- Sat Jan 08, 2011 6:01 pm --
I should also add that I did not apply for the card in order to deceive CBP officers. I was simply a frequent traveler, for leisure (vacations), visiting friends and to visit my US bf and I wanted to expedite my border crossings. As am aware, the card is intended for frequent visitors to the US who pose a low threat risk. My visits have never been more than 3 weeks in length, I have always entered with a return ticket in hand and will continue to do so. I just felt the need to add that caveat in case someone took my intentions for applying for the card, the wrong way
