Yup but if don't delete system partition all app's remain so it can conflict with the new ones. This could be the explanation why after boot he was not prompt to sign up to Google. I allways use the manual wipe way and never had any issue.
You obviously didnt read my original instructions to him....
Process is
- Make sure you have TWRP installed
- Reboot to recovery
- Factory reset
- Wipe system
- Flash Rom, GApps, Magisk
- Reboot
Do it however you like, but I can tell you manually wiping data does cause issues. Ive seen and experienced it and I guarantee you Ive clean flashed more than anyone you know.