I was basing everything i'd said on UK law, like manakin said you need quite a reasonable amount over here to see them go for a prosecution, i'm not sure on the actual amount but something like 3-5grams would probably see you get a fine if its not your first offence and whether or not its in separate wraps determines whether or not they'll press a supply charge.
secondly where your friend keeps accusing you and you accusing him, over here it would just end there, there is no point in taking it any further because it'll never get settled, you can't find a guilty party if both parties blame the other side in this particular case, the police themselves witnessed your friend throw the joint so, he would automatically be the guilty party doesn't matter if he had 50 friends backing him up, doesn't erase the fact that he was caught with it and not you.
Its hard to say for sure what will happen because I and probably most ppl don't know your laws or how good/lenient your justice system is. again talk to your dad, if he really is a lawyer you won't get a better defence, the free one they provide you, won't be as good as your old man.