personally I think the best chance the French had was in 1866... if they had provided support they would have had another front from the Bavarians, as well as states like Hanover who were opposing Prussian efforts at unification. The Prussians would have had to deal with another major army the French) in addition to the Austrians and that might have enough.
However, the French didn't, and Prussia basically forced the German states that were opponents into line, making for a far more dangerous threat to France in 1870.
Given the commanders the French had, their lack of a realistic strategic plan, and the well developed Prussian plan (split the French apart and defeat them in detail) it would have taken a supreme commander on the French side with far more energy and foresight to prevent that. The French had a sick Napoleon III.