Don't let your mom near any of your snow stuff anymore. Read the directions for using the Nikwax and wash it/dry it yourself in a separate load. If your mom complains about you wasting a wash cycle for two items of clothing then ask her whether she cares more about $2 worth of water or you getting frostbite on the hill due to a $100 pair of pants that she tried to ruin.
BTW, wour mom is giving you crap. Every woman I have ever met knows and understands what its like to have a favorite clothing item ruined. She might not care (since its yours), but she knows that she messed up. Ask her if she'd be so smug if you put bleach in the wash with her favorite cocktail dress.