I would disconnect all pc's, unlpug the router completely and then connect only the power. Then use a clean laptop to plug into the ethernet, switch off wireless and set a new pw. Connect the router to the net and leave wireless off and disconnect all pc's. Leave it for a day or so and see if the router has been changed.
Then try the same thing but have his pc connected and the internet unplugged.
You should be able to find out whether it's malware, a remote attack or a wireless attack by doing this.