I'd probably describe the discarded shield as being made of wood, especially if I had placed it there specifically for the druid to use as an item of last resort. The player wouldn't bat an eye.
If that wasn't possible for some really weird hypothetical reason, I would go the Thorin Oakenshield route: I'd let the druid pick up a nearby chunk of wood or whatever, to use as a rudimentary shield.
And if that wasn't possible for an even weirder, more hypothetical reason: I would call a break, discuss this with the player, and devise an after-the-fact ramification that both of us can live with, then return to the game. It would probably involve loss of powers and atonement, and would become the premise for another adventure.
And if even that wasn't possible, somehow, for no reason I can even imagine: I would conclude the player is being stubborn and actively trying to break the rule for no other reason than to see it broken, I would call a break to discuss that issue privately with the player. The player would likely be removed from the group.
Thank you for the serious reply! (I hope you understand I'm trying to resolve in my head the "player agency" concern here.)
I'm envisioning a back-and-forth with the player, starting something like
"That shield-- your fallen fighter friend's-- is made of metal. But you spot a hefty wooden stool nearby that would make a decent makeshift shield. Will you use that instead?" If the player persists, it's deemed a serious issue, and the game stops to deal with it, first as a rules issue, then as a fiction possibility, and finally as a table problem. (Presumably, if the player just asks point-blank:
"What does my spiritual instruction tell me about using metal armor?" that goes to a table discussion, too.)
More abstractly, as I'm understanding it, an aim of the GM on this is to avoid the rule conflict, if at all possible. But if the player insists* on pursuing it (as a player misunderstanding, a PC's moral choice, or whatever), the table stops to hash it out first
outside the game, then jumps back into the game to continue the story itself.
Is that a fair take?
If so, then seeing it laid out like that, I think I finally grok the
what and the
how of it. (Still a little fuzzy on the
why, but that's a probably just a personal taste thing.
)
* In hypotheticals, I personally think it's easiest to always assume the GM and player are sincere, or at worst "not a good fit for this game." I mean, booting a problem player who's just being a donkeybutt is always
a legit answer.