For me? The mariner's armor may have actually been made of fish scales. But the real issue is how much you wan to be a stickler for rules. The rule, to me, is simple. It has nothing to do with proficiency, it's a taboo.
So I would have just done a timeout with the player and tell them that: it's a taboo to wear armor for druids, when you pick this up you realize it's metal so you won't wear it unless you want to no longer be a druid. I've never hit this situation but there are (incredibly rare) times when I have to remind players what the rules are. On the few times it's come up there wasn't any real pushback other than to perhaps clarify.
How easy you want alternative armor to be is up to the DM and group. Wooden shields I have no problem with (although they'd have a chance to break if not magical), I would say that exotic material armor would be similar to adamantine or mithral. Difficult, but not impossible to purchase in my campaign.