From what I get from the OP, it sounds like he's not asking for help getting her motivated but how to help her in the gym without ruining his own work out. You know, he's trying to work out, but having to take a 5 minute break in between every set to show his fiancee what to do next.
Anyway, if I'm right about that, you need to think this out ahead of time... figure out what you can both do as close as possible in the gym so you aren't running all over the place in between sets. Easiest way to do it is set up in the rack, roll over a free bench and a set of dumbbells, and do everything all in the same spot. You can squat, bench, row, press, chinups, etc in the rack.. or do the same DB work she is doing. If your fiancee someone who is just doing weight training for general fitness and to supplement marathon training.. she can get everything done with dumbbells (lunges, stiff-leg deads, rows, press, arms, etc). Do any machine work you want at the very beginning or the very end of the workout and try to get most of your workouts done in the same spot and it's easy.
Either way it won't be more than a few months if you train regularly together that she will pick up everything and not need to be babysat and you can focus on your own workout.