The proposition that money has no real effects in the long run, referred to as the principle of long-run neutrality, is universally accepted today by monetary economists. When Friedman wrote, however, the conventional view held that monetary policy could be used to affect real outcomes - for example, to lower the rate of unemployment - for an indefinite period.
The idea that monetary policy had long-run effects - or, in technical language, that the Phillips curve relationship between inflation and unemployment could be exploited in the long run proved not only wrong but quite harmful.