On a typical (well regulated) piano with average key lead placement, a key around middle C will contribute around 17-20% of the inertia on a downstroke. The situation changes a bit when studying the key return to the up position, but I have the feeling that in this case the pianist (who is dealing with arthritis) is more concerned with pressing the keys down than the speed of key return.
In this case, the biggest improvement would come from reducing the action ratio, followed by hammer weight reduction. If the pianist mostly plays softly, I would even consider leaving the lead in the keys as is. That would reduce the balance weight (which matters more for soft playing).
For reducing the action ratio, the previously given suggestion with half punchings at the balance rail is probably the most pragmatic solution.
If you do reduce the action ratio by a significant amount, consider if the pianist would benefit from a smaller blow distance so as to keep the key dip the same as before the modification. The down side is, the piano will not be as loud if you reduce the blow distance and she might try to compensate with more force. Hopefully, she can be happy by playing in the lower dynamic range.
You should first make sure that the action is very well regulated before proceeding with any modifications. Measure the up and down weights to see if there are any friction problems. Adjust for the smallest possible let-off distance that will still work reliably. Make sure that the repetition lever springs are only as strong as they need to be and that the hammer drop is not excessive. Make sure that the jacks are aligned properly with the knuckles. Check the key dip. Some pianists say that the action feels lighter if it has less aftertouch, but this is highly subjective.
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