The euphemism known as an IRS audit would interfere in his life if an exchange into or out of USD took place. Anyone can attribute value to anything. That doesn't make it valuable. Taxation here would require provable gains or losses, as with USD. The transactions of people exchanging only in [bitcoins, metal, coffee beans, rocks, fancy/worthless paper], not in USD, do not imply that any taxable "gains" or "losses" occurred. If it implied this, there would equally be a lot of losses claimed.