Not a lost cause at all, sewing machines are one of the better documented categories because so many survived and people kept using them. The serial number is your key, on most makes it dates the machine and identifies the model family. ManualMachine https://manualmachine.com/ lists the main sewing brands including older machines, and threading diagrams are usually in the user guide rather than the service document. Worth checking whether the attachments are original to that model, boxes get mixed over generations and half of them may belong to something else entirely. Since you are doing the searching, print the threading diagram rather than showing it to her on a screen, it is far easier to follow with the machine in front of you.