Are you kidding me right now?
I feel like you must be.
It doesn't matter if they can wear metal armor. They will not wear metal armor.
Fighter: "Here, wear this half-plate because you're proficient."
Druid: "No. I will not put that on. It is unnatural for man to be clad in metal."
Whether or not he physically can wear it is irrelevant, because he won't put it on.
Eh, it depends how closely your table follows the fluff attached to the classes. There's that whole spectrum from strict adherence to total disregard and the OP's table might fall anywhere in the mix. Suggesting half plate because there's no mechanical penalty is a reasonable enough thing to do.
My homebrew: A list of 5e homebrew magitech augments found here, a homebrew race of devil-toads here, and the Professional, an all mundane support class. Criticism is much appreciated!