
I think you can but you need to find yourself a geek, the police when they investigate always take away computers to study the hard drive.
I feel for your friend, if she's not the paranoid type then she's getting that gut feeling for a reason. My first husband was like this, the history used to get wiped every day. One day he forgot and I was dismayed at the type of p**n he was looking at. I had my gut feelings about him too which proved to be correct.
I'm not a prude, but when it's secretive and the type is just nasty and sad then even if there is no affair it is very damaging to the relationship.