Lots of backing soda. Before use just brush it out.
We had some boots which had a bad time with some mice in a Munich basement storage area.We actually washed them in a mild laundry detergent, and rinsed multiple times, then dried in the sun.Helped lots. Used them another 5 seasons.
There is some stuff called "odor eaters" for shoes. Who knows. If nothing works, you might just have to avoid skiing with that friend.