Not paying taxes on a publicly-ledgered asset with a well-accepted monetary value that you then disclose to a government is the height of stupidity. If your friend’s home country has any tax treaties with the United States, I’d encourage him to get his paperwork in order with them.
And yet many people hold gold, or other precious, non USD materials, but nobody freaks out about their about tax fraud risks.
If the government starts coming after everyone's non USD commodity holdings (which, to be fair, isn't entirely unprecedented. Owning gold was illegal at one point), then we have much bigger problems than just as it applies to the comparatively tiny crypto market.