What data does he expect do get back?
TWRP mainly does a backup of the current state of the installed OS. Everything in “Internal Storage” (pictures, downloads, etc.) is not backed up by TWRP, and there’s a clear warning about that while backing up the data partition.
If no other means of backup concerning Internal Storage were taken (e.g. copying everything to a PC via USB or syncing the data to the Cloud or to a PC), then this data is now lost after wiping the phone.
Anyway … I wouldn’t restore the data partition or any other OS part of one Android (Fairphone OS or Open OS) to a different Android (LineageOS), and also not from Android 6 (TWRP 3.0.2 implies Fairphone Open OS prior to 18.10.0) to Android 8.1 (LineageOS 15.1). Not a good idea conceptually.
Here’s a backup guide for future reference … ✏ Creating a backup of user data using TWRP community edition
If all else fails, a signed TWRP image can be booted without installing it …
https://forum.fairphone.com/t/howto-use-twrp-without-flashing-it-e-g-for-backup/23747