It is fair to assume that a President Le Pen, lacking the legal ability to withdraw from the EU, will follow the Polish and Hungarian playbook and try to render it powerless from within. Given the importance of France to the EU, she could succeed. What then?
Remember, the EU and NATO were created largely to keep the Germans under control. If the two organizations are effectively neutered, the Germans–now more economically and politically dominant in Europe than ever–will have some important decisions to make. One can imagine them developing nuclear weapons and becoming more diplomatically aggressive in order to fill the new security vacuum. Would that outcome be welcomed by the French?
France, be careful what you ask for, because you might get it.