I do realize that when the player agreed to play in a game where the rule was druids wont wear metal armor then yes, that was a choice they made.Yep. The halfling rule still works completely differently from the druid "rule." One says halflings only have a walking speed and shows them built like small humans, physically incapable of flight. The other fails to say anything about druids physically being unable to put on metal armor. There is no explanation in 1e, 2e, 3e or 5e that says why druids don't wear metal armor, except for how it affects their magical abilities(1e and 3e). If you can point me to something that says that druids are physically incapable of putting on metal armor, your equivalence will cease to be false. Can you show that to me?
You do understand that "won't wear" means it's a choice they make, right?
If they preferred to play in a game where the rule was "druids will wear whatever armor they care to, they should play in those.
See how that works?