I'm in a bi-national relationship with a Canadian and disclosure would be very bad. Its a reason that a border agent can use to say you "may" overstay your visa. Its a tie to the country you're visiting. Until I got nexus, I was coming to visit friends, and then acouple of agents asked "are they more than friends?" That is what pushed me to get nexus.

I'd recommend a nexus card. She may have to provide more financial/job info to show she has ties back to Canada.