Because the snowboarder stands still, its kinetic energy is zero. But it has lots of gravitational potential energy. Its quantity depends on the level which we suppose to be zero (usually sea level but any may be chosen).
When the snowboarder descends, his potential energy decreases (with respect to any fixed level). And it is spent on friction (thermal energy) and on speed (kinetic energy).
Inside himself snowboarder also has chemical and thermal energy.