A man is standing on one side of a boat and the boat is stationary. We ignore friction between water and boat (and air friction). Thus there are no external forces on the man+boat system. So momentum is conserved, and centre of mass does not move. I understand that if the man moves to the other side the boat moves in opposite direction that the man moves in.
My question is, if there's no friction on the boat, once the boat starts moving in the opposite direction, what force exactly stops the boat from moving once the man has reached the other side of the boat.