thank you for pointing out the documentation issue; the In2_Membrane_ElastMat.bendThickness said it was for CST (Constant Strain Triangle) stiffness computation, but it is really used for DKT (Discrete Kirchhoff Triangle, the bending element). I fixed the documentation now.
So halfThick is the geometrical thickness, but you can override it for CST and DKT computation using In2_Membrane_ElastMat.thickness and In2_Membrane_ElastMat.bendThickness. This is more for development/debugging (since that value will be applied to all Membranes in the simulation), so the defaults are NaN for both of them, and 2*Facet.halfThick will be used.
FrictMat is a ElastMat (it is a subclass); as internal forces of memberane don't depend on its frictional properties, only on the elastic ones (contact forces are consumed in internal forces evaluation, but they are computed elsewhere -- you have Cp2_FrictMat_FrictPhys Law2_L6Geom_FrictPhys_... in your contact functors, that's where it happens). So it is correct that you use In2_Membrane_ElastMat; there is actually no In2_Membrane_FrictMat specialization of the functor.
If you find some further inclarities, let me know, it is always to good to polish things. It might also happen that you discover some bugs in the code (though I hope not :) ).