Claim very promising. Winch grooming during operating hours is the exception.
During operating hours the slope operator must not in principle carry out winch grooming. A steel cable strung across the slope while regular slope users are out is an atypical hazard in the classical sense. The breach of duty is regularly to be affirmed.
First steps: obtain the slope-rescue protocol, clarify the slope-closure status at the fall time (was the slope really open under the operator's announcement?), secure the lift ticket as proof of contract.