Here's the Washington State law:
R.C.W.79A.45 030(1)- “In addition to the specific requirements of this section, all skiers shall conduct themselves within the limits of their individual ability and shall not act in a manner that may contribute to the injury of themselves or any other person."
If you wish to pursue the matter, you or your lawyer needs to check the law in the state where the injury occurred. Gather the facts, cool the emotion, and decide if it will be worth proceeding.
79 year old bones break more easily and heal more slowly than younger bones. Mainly hope that your dad heals and gets back on the hill next year.
File that one, and every other such state law (except Colorado's lunatic ski law) under the "been there, done that, tried, and failed."