Originally Posted by
BearX220
As it was a weather-related cancellation I am going to say there's no case. Your sister would have gotten to San Diego with Frontier in the first available seat after the fog lifted, but she chose to go her own way via Vegas. She can request a goodwill refund / credit for the unused portion of her ticket, but you won't see the airline paying for her rental car, hotel, etc.
Agree, she opted to go her own way, no chance for compensation. However I would try it.