They'd very likely provide arms and aid to the Italian government but I doubt they'd openly intervene without a formal request from the Italian Government. This is also a huge political win for Mussolini and the fascists who now have direct evidence to point to when harping on the socialist threat.
I also can't see the Socialists getting too far, even in a major uprising since by 1921 Mussolini's National Facist Party had managed to grow to more than 300k members largely as a reaction to the Beinnio Rosso and by 1922 anti-Fascist sentiment seems to have been lukewarm at best. If the Socialists go all out in rebellion I suspect that the counter-reaction would have been even larger than OTL from an Italy that was largely unsympathetic to their cause.