Hello
@SummerRunner !
Welcome! I'm new here too, just came a few days ago, but have found nothing but kindness, politeness and excellent/valuable advise from all the SkiDiva ladies, so I think you'll love it here too!
About your friend, here is my honest opinion, based on my own experiences:
I grew up in the world of classical ballet, which is an extremely strict world, requires full dedication, discipline, patience and being in it since first years of life, it creates extremely ambitious, competitive individuals. I am one of them, and an angry Russian director still lives in my brain telling me to work harder, do better, and excel always. I'm thankful to this upbringing since it taught me many excellent character qualities I apply in every aspect of life and my angry Russian director was actually kind pushing me because she believed in me. Ignoring me would've meant I didn't have a chance so I'm eternally grateful.
HOWEVER
You're in a setting where you are going to ski with friends. Your high level very skilled, competitive skier friend accepted to go skiing with her peers who might not be as skilled as her or not approach the sport in such a competitive, ambitious way as she clearly does. I think her attitude and behavior is quite wrong. As the most skilled person in a group she was not forced to join, went out of free will and in which she apparently has friends in, it is my honest, rational opinion she has two options here.
FIRST (most obvious) she behaves like a FRIEND first, puts her competitive, high skilled ambitious self on hold, and does nothing but encouraging, giving wise advise, constructive critique and offering support, which is, I believe, the "job" of any person who's worthy of being called friend. She roots for you, celebrates your improvement and helps you in areas you might struggle. Yes, she's not your coach, however, nobody needs to be official paid coach, to give helpful, beneficial advise to someone who's less skilled at something you excel.
SECOND (unfortunate) she cannot deal with peers who cannot keep her same pace or not have the same advanced skillset as she does, it annoys her, she gets impatient and wants to use the time to do her own advanced/expert skiing. Good, so be honest and in a polite way, she apologizes to all parties in the group saying she will go skiing on her own and maybe later she meets the other peers for some schnapps.
NEVER, EVER does she have any right to demean, patronize, use condescending tone and humiliate her PEERS.
As a competitive, ambitious person myself (nowhere close to being expert/advanced skier, but I am one in my job/occupation), it is my responsibility to either encourage, support and lead my junior coworkers in a way they improve and excel too, and in personal life, if I accept out of free will to be part of a setting that not everyone will be like me, I'm also accepting a responsibility to behave well, get along and never humiliate anyone.
Proposed solution: If you believe this lady is worthy of keeping as a friend for whatever reason, I suggest an open, blatantly honest conversation is a very calm, polite tone, but explaining in detailed manner how this behaviour is unacceptable. If the lady appreciates your friendship as you appreciate hers, she will think hard, analyze and behave appropriately next time.
If not, it's best to keep in mind this unfortunate events with this person, stick with peers who would never treat you or anyone else this way.
I'm very sorry for the lenghty reply!