That doesn't sound normal. Many alcohol stills use copper (wikipedia has this bit of information: "(a copper still) removes sulfur-based compounds from the alcohol that would make it unpleasant to drink")

Yeah, that's why I'm scared of it now... any clue how I could test it?

What are the sides sealed with? Alcohol is a good solvent so it may be an adhesive flavor youve tasted.

You could check the density if you think its not actually copper material though.

There are copper testing kits for aquarium water, but your health isn't really worth risking with a $10 aquarium test. Throw it away and buy a new one, and research how to properly handle copper cookware.

A Moscow Mule is typically served in a copper mug, and I'd think the negative effects from the alcohol would be the primary thing to worry about.

The thing to worry about would be poisoning from copper toxicity (by misuse above, or by overuse of copper sulfate), or if made with beryllium, beryllium poisoning. If somehow 30mg/kg of copper were swallowed, it is potentially lethal. And if any food residue is left stuck to a piece of copper cookware, it should be thrown out immediately so it doesn't leech copper out.

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.

only water, and once bourbon.

Most current commercial mugs have a lining in them to prevent any leaching but still allows you to maintain the look and also the copper to help with temperature.

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.