eggshells isnt enough to press charges. you have no proof it was
him. 
you need an eyewitness at least. just be glad you caught it in time and that it was very cold outside because egg yolk is very acidic and can ruin your paint. it sucks, its happened to me before, but i would just let it go. if you retaliate then things will probably just escalate and you may come outside to find your car banged up or spray painted etc... i would definitely tell your friend about and say that because of her actions this is what happened to you! it may sound like the "puss" way to handle it but drama only begats more drama.
now if youre sure it was him and you can take him, then confront him about it. just be prepared for any and all consequences that may follow.
again, to reiterate, be glad it was only eggs.
