Let's attach some numbers to this problem and find the minimum horizontal force that the climber must exert on the wall to keep from falling.  The climber's mass is 55kg and this center of mass is located in the middle of his torso (0.25m below his shoulders and a horizontal distance 0.20m from the wall against which his shoulders are pressed).  The fissure has a width of 1.0m.  The coefficient of static friction between his shoes and the rock is 1.1 and between his shoulders and the rock it is 0.70.