You can't backup a remote PC over the Internet. Even though her PC is part of your Circle, by virtue of the fact that it is activated with your subscription LiveID, centralized backup can only be performed for PCs on the same local network. She will need an external drive or CD/DVD to backup locally and can't use Centralized backup.
By the way, you are not computer illiterate, since you have configured backup fine for your PC!
-steve