Proof, no. Evidence, yes.
The evidence is that Serbia has been completely and inextricably silent about France and Germany's "go to the UN" double-standard.
And about that dearth of news, there is no dispute.
Why? What possible motivations could there be for Serbia to be complicit (with their silence) with France and Germany's newfound desire to take war-issues to the UN?
... and then the Serbian PM gets whacked.
Go figure.