I don't have a Nexus card, and I probably won't get a Nexus card because the few times that I have had to wait more than a half hour the Nexus pass would have been useless (traveling with people who would never have a Nexus, spending more time on the crossing before I even got to the border/where the Nexus lane starts). I know a few people who have a Nexus pass, but they travel to the U.S. a lot more often than I do (like every day). I must admit the part about your friends telling her to get a Nexus card already hit a nerve with me because I know a few people who act super superior because they have have one (I'm not saying you or your friends are acting like this) and have such an attitude that I don't have one. And its not like I travel with them, so its not because I am inconveniencing them. (Sorry, I had to get that off my chest)
If she demanding frequent trips to the U.S. but won't get a Nexus pass I would be blunt, and say that you really don't want to wait in line. And even if you rather not make the trip with her, I would just tell her nicely that although you would enjoy the trip with her, you rather not have to be stuck at the border.
I do have to ask, how long are your waits usually? Because my last shopping trip to the U.S. I didn't have to wait long. On the U.S. side I was behind one car, and on the Canadian side I was behind two car. Now I did go during the week and it probably wasn't the busiest hours, but if I had to wait two hours at the border I probably wouldn't go over as much.