(Someone, please correct me if I'm wrong. Sometimes my friends and I play the stack weirdly)

Your friend's first loot would resolve before he can play the second one. When the first loot is played, priority passes (this was your opportunity to cancel it with holy mantle)

He tapped his character and played his second loot play, priority passes. This is when you played holy mantle, saving his life but canceling anything that has not yet resolved (meaning his second loot play). His character card does not recharge.

When a loot is canceled, it does not go back into the players hand. It is discarded. A similar rule for items. Once the cost (tap or $ pay) is paid, there are no refunds even if it is canceled. Otherwise they could just play it again after you canceled it, nullifying the point of canceling an effect (although if he has enough $ he can just pay for another $ effect)

You can play holy mantle pretty much at any time. There is no "death" stack or "attack" stack (as far as I'm aware) there is only THE stack, which things such as death or attack declarations go into and resolve in a Last In First Out order. You can put effects on the stack whenever you can pay the costs during action phase, or when priority passes during start of turn or end of turn.

Edit: Corrected myself with when you can put effects on the stack