This is a total dodge. The question pretty clear, and the situation isn't especially fringe: the player has simply decided that their druid puts on ordinary metal armor.
So what does the GM do in that case?
The answer to that must be either (1) say "no," which is completely RAW but also obviously bars player agency on this issue; or (2) say "yes" and then follow up with a non-RAW ruling as a ramification (which might be nothing, or exploding the druid, or ejecting the player, or whatever).
Given the RAW as is, the GM has only those two options.
No. Hear me now and believe me later- this situation is solved in the only proper method for resolving disputes in D&D, the age old legendary game that channels nothing but your sheer will to dominate. File this under -
You come at the King, you better not miss.
A player who is asking to don metal armor as a Druid is not just asking about the rules- they are challenging your authority to run the game. Do not given into their so-called "player agency" whines.
Few people understand the psychology of dealing with your garden variety rules-lawyering player. Your normal DM will panic and immediately start throwing chain mail and plate mail and random pieces of metal fo the Druid when the DM hears the rule lawyer clear their throat and say, "AK-SHUALLY there is no penalty for wearing metal armor" ... and then the DM will start apologizing, begging for mercy.
This is wrong. It arouses contempt in the player's heart. The thing to do – when you're running your game and the player suddenly decides that they want their Druid armored up like some sort of Iron Armadillo – what you want to do then is establish your authority through the grand old game of ¿Quién es más macho?
That's right- this is for all of the DMing marbles. Both of you take a d20. And roll.
The lower number drinks that number.
If you tie, you have a roll-off, and the lower number drinks double the number.
Repeat until one person falls beneath the table recanting their heresy, to the extent they can still talk.
The victor ... is the DM. And that will be you. Keep smiling. The idea is to show the rest of the table that you were always in total control of yourself and your game – while the Rules Lawyer lost control of everything.