He already spoke to the police, then cleared his browser later. That means he knew there was some form of investigation already underway at the time he deleted the browser history.
Unfortunately, I think that means intent doesn't matter now, except when it comes to sentencing. It was evidence, by not preserving it once he knew there was an investigation, he fell into the trap. He may have been ignorant of the trap, but he stepped in it.
That sucks. But I don't see a way around them getting him for that.