DMH-Online

NWScript:

// cs_leave_cell makes the PC an enemy of the guards when they exit a particular
//      area. The script is reversed by calling cs_enter_cell.
void main()
{
    object oPC = GetExitingObject();
    if (GetIsPC(oPC))
    {
        object oGuard1 = GetObjectByTag("GUARD");
        SetIsTemporaryEnemy(oPC, oGuard1);
        SendMessageToPC(oPC, "WATCH OUT FOR THE GUARDS!");
    }
}

4. Make this the onEnter script (named: cs_enter_cell)

NWScript:

// cs_enter_cell makes the PC an friend of the guards when they return to an
//      area. The script is reversed by calling cs_leave_cell.
void main()
{
    object oPC = GetEnteringObject();
    if (GetIsPC(oPC))
    {
        object oGuard1 = GetObjectByTag("GUARD");
        SetIsTemporaryFriend(oPC, oGuard1, TRUE, 15.0);
        SendMessageToPC(oPC, "You should be safe again.");
    }
}

This works better if the guard is patrolling some waypoints. They should go back to their patrol when they are friendly to the NPC again.