France doesn't need Prussia's success. Prussia, with its divided territories, geographical vulnerability, weak Navy and relatively low population, was hanging somewhere between being a great power and not being one - and through Bismark's actions (among other things) rose to actual undisputed great power by uniting most of Germans into one state. France doesn't need to do this dramatic jump - France has been already consolidated the way divided German states weren't. Bismark-Era France needs peace at home, divided Germany in the East and maybe some counterbalance for British influence overseas. It does not need to double or triple its territory.
So if Bismark can placate the people with something akin to his Sozialgesetze, keep southern German states from rallying to Prussian banner and cobble a coalition together with southern Germans/Austria against Prussia and avoid pitfalls like his anti-Catholic campaign - he can save France without anyone noticing, so to speak.