He should simply state: Perhaps you should look up the Innocence Project or pick up the book by John Grisham titled The Innocent Man before you make an ass of yourself believing everyone who is ever accused of a crime is guilty and deserves no defense.

But by all means, be sure to point out that they are making asses of themselves, because certainly that's what they are doing.

I used to be a criminal attorney and this came up all the time. I'd say that a person isn't guilty until a judge or jury determines that. My job was to make sure the prosecution did its job.