I believe they don't need to make an exorbitant offer like this. The French are probably really afraid by this point and are already preparing to join the German war-effort.
I bet that right now, France has heavily fortified its border with Spain (as well as the coasts, to stop any British or Drakian naval attack). France is also likely in war economy mode (with all available men being called for training every month, the industry focused on military production, etc).
Moreover, France could help without declaring war on the Draka by
1 ) Providing full lend-lease to the Germans and their allies, and use the French industry to support them
2 ) Let Germans do all their naval and aerial trade (outside of Europe) under French flag
3 ) Ostensibly prepare for a Sealion-style operation (by concentrating and training troops in Normandy, preparing transport boats, etc). The plan might not be realistic, but it doesn't need to be. It keeps Britain guessing "Will France declare war and invade", and forces the British to keep more troops at home. If questions are asked, say "It's a military exercise, just in case".
4 ) Concentrate troops on the Spanish border, so that the Draka have to guard Spain more heavily.
5 ) Send a legion of French volunteers (like Sweden in the Winter War OTL).
Doing all of that (without actually joining the war) would create an important nuisance and threat to the Societists, and force them to either tolerate it, or declare war on France. Which would likely mean having America declare war too. France just followed the old French-American Alliance Treaty of 1778, and declared war on Japan in solidarity with the USA. The USA would feel politically and morally forced to return the favor if France is invaded at home... Not to mention they don't want the Societists taking over the whole Europe.
