Extra wrinkle here for you wise folks to consider:
I'm not so much worried about the DR reduction of 10 points (I'm not that much of a tightwad that I can't give a player the benefit of the doubt on a potential 10d6 fall), I'm interested in whether this uses up the hundred or so points of absorbed damage that the spell conveys.
But I guess the two go hand in hand? If I let the PC reduce the spell damage by 10 points of DR, then their spell buff is going to absorb the rest. They could walk away unharmed but that spell is going to take a beating.
I do appreciate all the time and energy this question has garned though. I'm not asking just to ask, I forsee somebody with a high level stoneskin falling a hundred feet next game session.