To say that sounds fishy is an understatement.
There are ways to block a website (or that websites
IP address) via the hosts file on a machine. That is not an easy thing to do however, and would require editing some of the system documents. It's a trick used by system admins to block pages like say ESPN.com from their employee's to get more work out of them.
I sincerly doubt this little trick was used. It is not very easy to do, and requires an understanding of how network DNS requests are routed that the average 13 year old in all seriousness would never have.
It sounds like his limited understanding of computers is making the situation worse. He has propbably done something faily benign, and because he does not understand how it works, he just chose the easiest way to phrase it to your mother in law.
Kids who think they know how to mess with a computer often do the worst damage by changing settings to do what they want and not understanding why that change worked, so they can't undo it after.
I'll bet dollars to dounuts this is a nothing more than a change in the internet explorer connection settings (Right click on the IE icon on the desktop to view them) that is cuasing problems.
In order to really dig in and help, we need to know the following:
Her connection settings for IE
The modem or network connection settings from the connections folder in the control panel
If she can access other web addresses. (<--Answer this first)
God my spelling is awful for someone with two college degree's