I don't think of the Stiffness Factor for Friction in terms of units. If I would, it would have to be (1/length) since we use the load (mu*N) with this term to give stiffness. We add this stiffness to the dynamic analysis stiffness matrix to "simulate" the effects fo friction at the support by adding two orthogonal restraints perpendicular to the normal load (N) with this stiffness.

My focus is on natural frequency calculation. I see this more as an on/off switch rather than a magnitude thing and enter 1000 for this value. In our seminars, I have others enter 200 or 500 and the calculated frequencies vary only slightly. I guess if you were expecting this "friction restraint" to carry some dynamic load, you might have to "fine tune" your value to get the numbers you want. But if you want to push the program around even more, you can enter these "friction restraints" by hand as snubbers instead.