Just to be clear, tether has nothing what-so-ever to do with any kind of an internet connection, security issues or anything even related to hacking.
If someone you know is seeing the use of his/her personal information somewhere other than where the user expected this information to be available then his/her account has been compromised. The good news is that changing the password to that account will fix the problem TODAY!
You need to get more details about what is or is not being accessed to understand just how serious the problem actually is. If all of the pictures on my computer are being shared Nationwide I might actually be flattered. If my credit card information (never found on my computer) or my social security information (never found on my computer) is being shared I would want to delve deeply into what exactly was going on.