I have to agree with Perseus. It would have taken until September 20th before the French would have been in any position to attack. The war in Poland was virtually over by September 17th and any attack by a French Army geared to the defensive would have been a big mistake. Furthermore, General Kutrzeba, Director of the Polish Military Academy, had stated before the conflict that Poland would have to rely on her own forces for 6-8 weeks to defend any German aggression, even if the French reacted as quickly as possible. As history records, they lasted 18 days.
The French, as it happens,
did attack Germany on September 7th in 'Operation Saar'. It was a complete fiasco and can't have done anything to improve French confidence in their own offensive ability.
http://militaryhistory.about.com/b/a/207534.htm