Has your dad responded? I would want to make certain that he received a citation from the court and not one of those 'courtesy notices' that invites you to respond- and the response itself is thereby what 'traps' you into acknowledging the receipt. Many are intimidated and just respond, but I encourage you to explore this aspect...
As pointed out above, your dad needs to be careful not to do anything that forces him to have to answer a question of "do you know who was driving?". The contents of a TBD can in fact be a waiving of your right to not testify. (Hence camhater's advice to not have anything in the TBD that is a 'first person account'.)
I agree short and sweet: "Here is my drivers license, I am not the person in the photograph you provided in the mail, I am therefore not guilty". (none of these statements refer to the day in question, nor are they statements about his actions on that day.)

