I wouldn't plow again... as the others said, she may sue.
But here is what I would do... I'd send her an apology note with a list of places she can pay to plow her drive. I'd just like to make her realize that he was doing it out of kindness and that really, she is just being a miserable person in all of this. Kinda a farewell and smother them with kindness kinda thing. I mean, I wouldn't get sappy about it, just a "I'm sorry that you aren't happy with your freebie plowing, I thought that I was doing a kindness. Since you probably still need someone to plow, these are some for pay plowers that you may want to have lined up next time you have too much snow".
Today my neighbor brought over some organic strawberry smoothie for me... good neighbors are awesome.
Today my neighbor brought over some organic strawberry smoothie for me... good neighbors are awesome.