Answer:
Part a)
When there is no friction then acceleration is

Part b)
if there is friction force along the inclined then acceleration is

Explanation:
Part a)
As we know that the skier is on inclined plane
So here if there is no friction then net force along the inclined plane is given as

now acceleration of the skier is given as




Part b)
if there is friction force along the inclined then net force along the inclined plane is given as

now acceleration of the skier is given as




Yes, our friend is right, because there is no contradiction to the law of conservation of mass in the above equation. It just the mass of the product is equal to the mass of reactants.. and that is shown in the equation you have presented earlier
Net force can help slow down or stop the motion of an object.