They could be, the backup program or service won't know what's good or bad. The good news is viruses are always programs, and she'd have to run a program to get infected. Occasionally a data file can contain a virus, but it's rather unlikely. Backups don't typically include programs, just the user's unique files, so she probably is not backing up viruses. Especially since she's on the Mac, and the Mac doesn't have any viruses that are in this data/file format.
It is something to be aware of, though. Whenever reinstalling an operating system and restoring from a backup after being infected, it's generally recommended to do a virus scan because there is potential for restoring malware.