I think if the champion knows there is active combat, even if he doesn't see the ally, if both the enemy and the ally are within 15 feet, the champion can use his reaction for the ally to gain resistance to damage, another thing is to attack the enemy If you don't know where you are, you can't make the attack, but for damage reduction the requirement is clear, that both the ally and the enemy are within 15 feet, making it a kind of magical protection that grants resistance to damage, the champion does not absorb damage to himself. If you need to see the enemy and ally I would specify it as they do in other reactions (http://2e.aonprd.com/Feats.aspx?ID=550)
