I think that it would all depend on an end to the obsession of the cult of the offensive.
Which was not suited to either to manpower, geography or possible enemy.
The tough part is that the lesson you want the French to learn was true largely only in France and Italy, where terrain and force ratios made attacking harder. Even in these theaters, the Germans often took no heavier losses on the attack than the defense. In the east, there were hugely successful offensives, along with ones that were poor decisions to attack. When technologies change, it is hard enough to learn the general lessons before you fight the war. Even harder to learn the more specific lesson.
And it was only about a 20-30 year period where defensive warfare was supreme. Just as the machine gun and men-to-front ratios made the offensive difficult, the airplane, tank, and truck made offensive again the best option.
And then you get to even more profound changes. If the French really understood the losses and resource consumed by an industrial war, they likely change their diplomacy. And being a democracy, this knowledge has to be shared by some extent by the public. Which means the other foreign powers also will have access to this information. People like to talk about the red/blue uniforms, but the change was blocked by politicians not generals. Seems like Germany only change over within 10 years of the war. A-H was in the process of changing. The need to attack is driven by the need to reclaim A-L, outrages in A-L, and the glory of Napoleon. Once you see a war of maneuver, you end up with lighter equipment.
Now I think there are many small POD that give you a better French army. But going to full defensive mindset (we sit behind the fortresses in WW1) is a very hard and complicated POD. It almost certainly butterflies away WW1, and failing that WW1 as we know it. For example, building a large set of fortress lines west of A-L and Belgium means the Germans likely have the option of War Plan Attack Russia First still around. If this is true, the Germany will likely utilize it, since this is what the Kaiser wanted, but could not. So German does not declare war on France, but instead France declares war on Germany. And if France stays on the defensive, how will they save Russia from the mauling Germans. Germany had 7/8 of their forces going west. It is probably under 5/8 if France sits behind defensive lines. There are just so many butterflies to the changes you ask for it you want the full "defensive mindset".
Now if you mean, "In 1915, France sits on the defensive but we don't change things prewar", you are giving the CP a win. The Germans were mauling the Russians who had been retreating for months. Then a combined Anglo-French offensive makes the Germans send over 300 battalions west. If the French are more passive, the Germans will keep advancing to the east in 1915 from August to snow fall. This will create some big issues for Russia as they German armies drive deep into what is now Belarus. The Alan Island chain of defensive is undefensible. German will have a lot more land to loot. Since the Russians did get some refugees out in advance of the Germans, there will be an even worse food situation for Russia. Even if the Russians have successes like in 1916 with Brusilov, ITTL, the attacks will happen several hundred miles farther east and be easier for the CP to deal with.
I know Haig and various French leaders get blame for so many dead. But all a passive set of leaders does for France is to lose the war. There were no good choices, only bad ones. You attacked at high losses and hoped to win (OTL and it worked), you allow the CP to win, or you make a peace deal on the CP terms which are likely harsh. You have to accept the CP offers of the time, not wish the CP accepts terms the Entente likes. Once mobilization started, all good options ended.
