If you don't have an intermediary server, you'll only be able to backup when the receptacle is running. This isn't unique to crashplan, but rather is the nature of direct computer to computer communications. Honestly, Crashplan is the easiest I've worked with.
Alternatively, you can setup an FTP server on their network reasonably easily, and backup to it regularly. This could be a special piece of hardware, like a $200 Synology Diskstation, or a Raspberry Pi with an attached USB drive. Or it could just be a regular computer, though crashplan might be a better option in that case.
You could also do something cheeky if your friend is within wifi range, like giving them a USB drive attached to a router that connects to YOUR Wifi network. Then back up to it as if it were local to your house. But, of course, you're not as protected in that circumstance.