Shared files in Box and TimeMachine / how to retrieve files from a shared folder

When someone shared a folder with you on Box and this person leaves the organization and her/his account is deleted, then the shared folder disappeared from my Box folder content. So, how do I back up shared folders in my Box folder. Apparently, "there is no need to back up files in Box". But when a shared folder is deleted, how do I access these files if the person who shared it with me is no longer able to access Box?