I don't see how a mirror would be that big of a shock, I mean they've all looked in still water and seen themselves before.
I think the bigger shock would be when they all get slaughtered by farmers clearing land for corn and soy beans. Since it's illegal for them to clear forest that's inhabited by aboriginals, a lot of the farmers will go kill them all first. It's really quite horrible what's happened to the aboriginals of the Amazon area.