Maybe this is a stupid question, but are you sure the pain is due to the large Q factor and not to the centering float mechanism in the SPD pedals? You might want to consider changing to free float pedals (Speedplay Frogs if you prefer mtb shoes) before tinkering with the crankset. This might allow your stoker to adjust her feet position to naturally compensate for the crankarm position. While I can't guarantee that this would work, it seems as an easy first move in trying to correct the situation. Our experience: my wife developed a knee pain after two seasons on SPDs, she changed to Speedplays and all became good. I continue riding SPDs without any issues.