If they have ever used that "remember password" feature in Google Chrome, you can get a list of sites they use, and all their passwords in just a couple clicks. The good news is that Google has admitted that this is how it's intended to work, and does not consider this a problem. They will not change how it works to protect you or your friends -- so go nuts.
Here's how you do it (in 15 seconds, without any special tools or skills required):
1) Open up their Google Chrome settings on your friends computer while they are getting a coffee
2) Click on the "Manage Passwords" link in the "Advanced / Passwords and Forms" section
3) You are done. Click the "show" button on each password
