Ok the middle finger was not the slightest bit mature.
Why does most people think that the only way to resolve issues like this is by talking big to each other and waving around their so called "male-pride" stupidity?
What is this? A world of apes? (< well duh)
Either talk it out calmly, or ignore him if he can be ignored.
If he knocks your girl over call the authorities on him for assault.