To clarify what has already been said, just drag and drop the .MOBI files to the DOCUMENTS folder
on the Kindle. Connect up the USB cable and it will show a new drive letter on the PC, open it up in Windows Explorer and drop the books in in the DOCUEMTNS folder. When finished use the Windows USB SAFELY REMOVE HARDWARE tool in the tray area to safely dismount the Kindles USB drive and unplug the USB cable. The Kindle grinds away for a bit (evedent by the swirly in the upper left hand corner) and then shows the list off all the books.
One you get past a pageful it may not be evident the new books are there unless you have the book list on the Kindle sorted by MOST RECENT FIRST.
Amazon has tons of books for FREE, just keep an eye out. Project Gutenberg has 30,000 + books all for free, just use the MOBIPOCKET format option near the bottom of the page.
The Whispernet will work IF she has the International Kindle, end then some Intl rates may apply even to free Amazon books, so be careful until you know what charges if any may apply.