Phones are encrypted by default, so no you would not be able to recover data after a factory reset. (Side note, this security hole use to exist and is the exact reason they made encryption the default/mandatory years ago.) The only way you would be able to recover data is if you have it backed up somewhere. Google Play will restore any apps and purchase, and saved files are often synced to the cloud. Your personal files would have required a manual backup.
To answer your last question, it would be best for your girlfriend to backup her data, sign out of her Google account and then factory reset the phone before you use it again. That ensures her privacy is protected and removes factory reset protection during the reset.