Coefficient of Friction of a KE-Work-PE situation
Knowing the spring constant, you can calculate the elastic energy at maximum displacement from the position where the spring is unstretched. Determine also the potential energy of the whole system (both masses) with respect to the same position. Calculate the total energy at each maximum displacements. Without friction and other dissipative forces the energy would stay constant. The difference is the work of frictional forces. Plot the energy in terms of L, the distance travelled. It should be nearly a straight line ΔE=-F(friction)*L.