My sympathy to your friend -- it must have been a horrible experience.

Since no one has said it, though, I will:

ALWAYS turn the dead bolt and put on the security chain when you're in a hotel room. It should become a reflex. At minimum, it will keep out the guest who was issued the wrong key, or the over-zealous maid. It will, however, ensure that what happened to your friend doesn't happen again.

This is a habit for me -- enter the room, put down the bag, lock the door and put on the security chain.