I actually think the Germans would think this is a good thing. They "know" they can beat France anyway - and they also only really associate socialism with being anti-war (if there was ever a peacenik faction in the pre-WWI Third Republic, it was the socialists) so they'd probably think a socialist France would be even less of a threat.
A good point, but in letting the revolution succeed Germany would also indirectly empower the large socialist demographic within their empire - a notion I imagine the Junkers and Kaiser wouldn't be thrilled by. If any kind of leftist crackdown is initiated in the future, thousands of German socialists could just relocate to Red France before popping up from another corner of the empire when the arrest wave stops.
Also, I suspect the French socialist will not stay anti-war for long - the revolution has be spread after all. Red France would begin to export the revolution across Europe and be a general pain in the butt for Germany and their puppet states. I can see Germany being lazy and not nipping the revolt in the bud early, but eventually there will be many loud voices advocating for a military intervention.