Good answers. Obviously working on quad and calf strength is essential. Half-squats, plantar flexion (weight machine or elastic bands,) and full squats if the snow is going to be deep! The only way, that I’m aware of, to train for altitude is to progressively work out at altitude for 10-14 days. You didn’t mention the altitude your client is going to be skiing in. Above 14,000′ definitely requires acclimatization. I like the water bottle-clock position routine suggested by Timothy. It’d be great for assessing balance skills. I hope your client has a great ski trip!