I think that schools will look at her skiing as a very good EC and recognize that it takes a lot ot time so will excuse her not having other ECs. However, if she’s not a recruited athlete then it is just another EC. Will her skiing make her stand out? Probably. Will it get her into college if she’s not a recruited athlete? Probably not.
It’s very hard for 16 year olds to make the decision to go all in for a sport. My niece was asked to attend a ‘skier’ high school where she’d ski every day and would have received a full scholarships. It was about 45 minutes from her home and she would have trained at her home ski resort. She was a jr. Olympian. She decided just to stay at her regular high school, train after school with the kids she’d been skiing with since she was 5 years old. She ended up in college in Hawaii and her ski racing days were over.