J.Quondam
CR 1/8
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.Why would you? Did you as player agree to play the game by the rules? Why would you intentionally break a rule?
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.