From what I recall, you can really only export "some" files and settings if apps allow it. Also, adb backup is deprecated, and not complete.
Otherwise, if your friend isn't opposed to Google backups, you might be able to use that and then restore once lineage starts up. Though that also means installing gapps.
Feel free to correct my assumptions there, anyone else ๐
Edit Just realized "user files", which you can adb pull /sdcard ./localfolder and then adb push ./local folder /sdcard once lineage boots up. That'll copy all downloads, pictures, etc off and then on the phone.