Answer
In terms of momentum conservation: Since no external forces act in the horizontal direction, momentum is conserved.
Because Freddy is falling vertically and has no horizontal momentum, the skateboard + Freddy, with its greater mass, should slow down after the frog lands.
This is similar to the situation in Figure 6.14 on page 99.
In terms of force: When Freddy lands, he speeds up to the right. This horizontal force is provided by the skateboard. By Newton's third law, Freddy exerts an equal force on the skateboard to the left, slowing it down.
