As others said, there would have been plenty of options to rescue the data or get the system running again - everything could have been done via the live CD.
But maybe Manjaro could prepare for similar cases that may happen in future, so it won't happen again. The problem here I think is a lack of directly accessible information what to do if a user wants to recover something. The live environments could be extended maybe by following options:
- Provide a "recovery guide/application" that covers the steps how to get help, backup data and rescue the user data
- Calamares could offer a "separate home partition" option if a user doesn't partition manually (AFAIK it takes everything for root) [more a preventive measure]
- Calamares could detect an installation that is about to be overwritten and could offer information / maybe an option to migrate /home ("Migrate your data")