A very urgent question

Statement 1 is wrong. Since the man and the rock are at rest, both of them have a net force of 0. Hence, by considering the force acting on the rock, the pushing force should be equal to the frictional force. Decreasing the pushing force will therefore decrease the fricitonal force.

Statement 2 is correct. Both of them have 0 net force. Consider the force on the man. The reaction force by the rock and the fricitonal force should be acting on the man. Hence, they are equal in magnitude and opposite in direction. Now consider again the force on the rock. It will be the pushing force and the fricitonal force. Hence, Fricitonal force equals the pushing force.
The reaction force by the rock on the man and the pushing force on the rock form a reaction pair. They are equal in magnitude. Hence, Frictional force on the man is equal to frictional force on the rock,
if and only if both the man and the rock are not moving.

The statement is wrong.

If the man decreases his pushing force, the firction acting on the rock by the ground decreases accordingly.

If the frictional force remains unchanged, there would be a net force acting on the rock when the pushing force is reduced. The rock would move towards the man, such situation is, of course, physically unrealistic.

2010-04-08 10:49:43 補充：
Your second statement is also correct. The whole system ( man+rock) is at rest, all forces acting on the system components (i.e. man and rock) must balance one another.