https://i.imgur.com/3k2QBQI.jpg
I love the thing, and I was drinking water out of it...
But one time I put alcohol in it... and I took a few sips from it and I almost immediately started feeling nauseous and weird... now I'm scared to use it, even for water.
You could check the density if you think its not actually copper material though.
If all he ever did was serve a cold moscow mule using his "copper water bottle", maybe the alcohol is the problem. But we only know he used it to drink water and alcohol. We don't know if it's lined, or if the lining was damaged, or if he used a hot or acidic liquid inside it at some point, or whether it has a corrosive residue.
I can't find a quick reference but I read a while back that the drink, possibly the acid from the juice, reacted with the copper to add a flavour to the drink. With the lined mugs you don't get that specific taste.