Actually, in the movie's time frame (1942) France had been defeated. After signing the armistice of June 22nd 1940 they were not enemies at all. Well, Charles de Gaulle and his supporters would have disagreed about that statement but that's another story.

Hopefully someone more qualified might expand on the terms of the armistice and the relations between Nazi Germany and the Vichy Regime (which administered the colonies such as Morocco), but the simple answer to your question is that your premise is wrong, they were not 'enemies' at that time and while France maintained a certain autonomy following the armistice, the terms signed kept them on a short leash and the way the Germans behave and are treated in that movie seems fairly believable to me.

Of course, that did not mean that there were no tensions and resentment (And acts of resistance, also another story). The scene in Rick's bar when the French clients sing the 'Marseillaise' to drown out the German's chant is a beautiful illustration of that. Did you know that beside the main characters, all the unnamed actors singing the anthem in that scene were French Expats? Many must have had families in occupied France at the time. I always find that scene very moving when I watch it.