ok, there may be other solutions, but i'd tale this one:

vec_set(dirVec, fighter2.pos);
vec_sub(dirVec, fighter1.pos); // vector to fighter2

to_angle(dirAng,dirVec); // recieve the angles

if (abs(ang(dirAng.pan - fighter1.pan)) < maxPan) { call BlockHim; }
/* if the absolute (degree) difference of the looking direction and the enemy's direction is smaller than the maximal difference, you may block him */

// not tested