In that specific case, just wait. France will look at Savoy's provinces and consider war themselves soon enough.
In general, there isn't much you can do. There are two options, but both are rather difficult to achieve.
- Destroy the alliance
The only way to influence a nation's opinion towards a third is espionage.
With 1.16+ you have to build up a spy network in Savoy using a diplomat, which allows you to conduct covert actions. From what I can see on http://www.eu4wiki.com/Espionage , Sabotage reputation is the only relevant one and it's available with Diplo Tech 21, so useless early on.
In an earlier version, sabotage reputation reduced the opinion of neighboring countries by 50, but required actually taking espionage ideas. So, also useless early on.
- Prevent France from joining the war
The AI will generally join a defensive war, so this is pretty hard to achieve, but always check whether France will join, when it's currently at war. The best chance you have is to ally with France yourself. Then declare another war and ask France to join. While you are in this war, you can declare on Savoy and France will not be able to help Savoy, because it is already your ally in another war.
However, all of these options take some time and it's likely the France-Savoy alliance breaks down on it's own before you achieved this. If you really want to try actively, go with the ally France route. That's a good idea in general, having the big blue blob as your ally as Switzerland helps a lot.