I suppose it's possible to reduce the size of your upper body while maintaining the muscle size in your legs. Usually when women diet down, their upper body gets smaller and leaner than the lower body. The lower body can be stubborn in terms of sizing down, while the upper body usually sizes down quickly.
I would suggest, as you have, to reduce the volume and intensity of the upper body training. Definitely still train everything in the upper body, but gear the workouts more towards toning than muscle-building... higher reps, more variety in exercises. Keep the lower body workouts as they are, and tighten up the diet.
One way to reduce upper body size and to get more definition is to diet down a little. Best approach is probably to diet down and see what happens to the entire physique. If she's like most women, she'll lose size in her upper body before her lower body just from dieting.
Is she dieting now? And how much cardio is she doing?