Friedman was a charlatan and monetarism was trivially debunked when he came up with it. The 60 years since have not been any more kind to it. Don't confuse stocks with flows. The whole idea of money supply as a useful metric needs to be put to bed; one would have thought the 13 years post GFC would have made that apparent.
(Of course, the Austrians have a peculiar notion that inflation can by definition only be considered as such if it's associated with increased money supply. Another reason to ignore them completely)
Between 2008 and 2020 M2 pretty much doubled and prices barely moved anywhere. This was intentional policy for the purpose of increasing inflation which stayed doggedly around zero. What does this tell us? Pretty much nothing because money supply is a useless measure.
What would happen if we halved it? Dunno, but to achieve that, stuff has to happen, and depending on that stuff, you might get price changes. I expect if the gov engaged in QT to achieve it, very little would happen.