Which part of the boundary layer(chordwise) contributes most to friction drag? fore? aft? Why?
It depends on the airfoil and the free stream conditions, but some general trends can be observed.

First, skin friction drag is dominated by viscous effects, so the laminar or turbulent quality of the layer is important. Laminar layers are less draggy that turbulent ones, mostly by merit of being thinner. This means that the leading edge will often suffer less viscous drag until the boundary layer transitions from laminar to turbulent, which may happen next to the stagnation point or further downstream. The layer will then continue to grow, so, for attached flow without re-circulation bubbles, the friction maximum can be expected to be near the trailing edge (not necessarily on it).

Second, for this viscous friction to be drag in the usual sense, it has to provide a force opposed to the direction of flight, so one would integrate its product with such a vector along the surface of the airfoil. This would mean, again, that the leading edge contributes less than its fair share, as friction forces directed up or down relative to the free airstream are not drag.

So the general answer would be that the aft part of an airfoil provides a larger friction drag contribution than the fore.