He doesn't. And proving hacking would be nearly impossible. A gifted hacker wouldn't leave evidence. Or at least no evidence that a normal investigator will find. And simply saying its a possibility doesn't matter without evidence. Thus, it's too expensive and time consuming to even consider.
What if he was hacked by a stupid hacker? Surely facebook keep logs which can be colsulted by the police. What IP address was the comments from? Where were you when the comment was made etc.
Interesting point, though.