The problem: Mom is hoping to be able to "breathe left and right." But when she gives it a try, she notices difficulty on her least favorite side.
I notice that there is asymmetry built into her kick. This video shows her old style of kicking:
Today's exercise: we try to swim with the sensation of the "front of hip and front of thigh" pressing downward on each stroke as if the front-of-hip were connecting to the track of a train. At present, we let the legs drag behind us and don't worry about the wide distance between them.
Today's goal: to swim with the weight shift of hips and more relaxed legs; the downward leg should find a moment pressing "in its train track" in good straight alignment.
Result: Although wide, a much more symmetrical "gait":
And by her next practice, a symmetrical and calmer leg kick provides effective forward propulsion: