Nobody does. Now why is that?

Friedman understood perfectly well that failure to increase the money supply in response to decreasing velocity was responsible for the Great Depression. He's sort of famous for that part, if I understand economic history correctly, it's why Bernanke apologized to Friedman's memory on behalf of the Fed. It seems odd to call them 'Friedman coins' when they depreciate automatically, but don't adjust to velocity and especially to changes in fractional reserve lending.

Probably executive order 6102 in 1933. By the time the government allowed private ownership of monetary gold again, it had fallen out of use as currency.