I would personally leave some files scattered throughout her folders saying 'sorry, but I got here by mistake. I'm sure you realise that's a bad thing what with all these personal details lying around and all, so maybe you should look into putting a password on your wireless access before someone uses this information for evil.'
With regards to equipment handed out by an ISP, it IS reasonable to expect that the ISP will deliver either a pre-secured router, provide a setup disc that does the job, or at least instructions on how to do so using big print up the top that says -WARNING- or -THIS IS REALLY BLOODY IMPORTANT-.
Many wireless routers were supplied to non-tech-savvy individuals to "get their laptop on the internet".
With regards to equipment handed out by an ISP, it IS reasonable to expect that the ISP will deliver either a pre-secured router, provide a setup disc that does the job, or at least instructions on how to do so using big print up the top that says -WARNING- or -THIS IS REALLY BLOODY IMPORTANT-.