I think I would try to explain to her that her cloud storage is not a backup. Yes, she will have copies of any files or images she has saved to the cloud…but her OS, applications and all data not saved to the cloud will be gone - music, video, contacts, other application data files, email, etc.
Some of that could take hours to reinstall while other parts of it would just be gone forever. She needs to understand the danger she is playing with here.
Plus, any backup plan that only stores you data in a single place is at risk of loss. You never want critical data subject to a single point of failure!
With an external USB drive and a program like CarbonCopyCloner or SuperDuper she can create an exact bootable clone of her system disk. Should that drive fail completely, she simply plugs in her external drive with the clone, boots up, and is back to work in minutes. The system drive can then be repaired at her leisure.
Note however, if that scenario ever arrives, she will then need another external drive to keep a clone of the first external in case it fails.