What happens when a Wandering Monster comes along that would ignore or befriend one player in a fight, but not the other? For instance, when the Wandering Monster is an Amazon and one player is female

When one player helps another, the monsters do not fight the players separately . . . so if one player is female, the wandering Amazon would donate a Treasure and leave without fighting. (Remember that Treasures always go to the main fighter to distribute according to whatever agreement was reached.)