Are US federal reserve notes money, or merely currency? I have heard from my friend that US federal reserve notes (aka dollar bills) are not actually money, but merely currency. He told me that money and currency are different, and that money is currency with some extra properties. That is, all money is currency, but not all currency is money. What is the truth of this? Is it the case federal reserve notes are merely currency, and not money as is usually thought?
Federal notes are both money and currency.
Currency can have two meanings:

In context of foreign exchange rate currency might simply refer to a system of money in common use in a particular country. That is, when discussing foreign exchange all USD, GBP, RMB etc are considered currencies.
Outside foreign exchange, currency means notes and coins in circulation. However, again currency is still money.

Money is more broad term in economics for a thing that serves as medium of exchange, unit of account and store of value (see Mankiw Principles of Economics 8 ed. pp 604). Consequently, money can be viewed as a sort of 'umbrella' term that encompasses currency but also much more.   That is all currency is money but not all money is currency. For example, money on deposit account will be considered money but not currency. Checks are also money but not currency. In addition, broad money will include also assets that can be easily converted to traditional money such as treasury bills.
However, most of money does not have special extra properties vis-a-vis currency. There is little practical difference between money in your wallet and money on your deposit account. Sure deposit accounts with debit cards might be more convenient (take less space, most cards now have contactless pay etc) but economically they are not special.  This being said when discussing broad money, such as T-bills, you could justify saying they have extra properties as well.