[MP2]

This may help:

pawn Код:

stock SetPlayerFacePlayer(playerid, faceplayerid)
{
    new Float:Px, Float:Py, Float: Pa;
    GetPlayerPos(playerid, Px, Py, Pa);
    new Float:fpX, Float:fpY, Float: fpZ;
    GetPlayerPos(faceplayerid, fpX, fpY, fpZ);
    Pa = floatabs(atan((fpY-Py)/(fpX-Px)));
    if(fpX <= Px && fpY >= Py) Pa = floatsub(180, Pa);
    else if(fpX < Px && fpY < Py) Pa = floatadd(Pa, 180);
    else if(fpX >= Px && fpY <= Py) Pa = floatsub(360.0, Pa);
    Pa = floatsub(Pa, 90.0);
    if(Pa >= 360.0) Pa = floatsub(Pa, 360.0);
    if(!IsPlayerInAnyVehicle(playerid)) SetPlayerFacingAngle(playerid, Pa);
    else SetVehicleZAngle(GetPlayerVehicleID(playerid), Pa);
}

However that works on angles, if you want to know if they are in the player's line of sight, you will have to use some maths with GetPlayerCameraPos and GetPlayerCameraFrontVector..