My best guess is that his local save file got over-written by an out-of-date cloud save because this wasn’t updated properly after that levelling session (maybe the system was shut down before the sync could occur?)
My second guess would be that his local save file got corrupted while it was being updated at some point - maybe a power interruption, a game/system crash, or a force-quit. As a result, the game has tried to restore the save from an old .tmp file or be re-syncing from the cloud save.
A third possibility would be if their game file had been set to read-only during the power levelling and had never actually been saved. That can happen if antivirus or anti-ransomware software treats the save file updates as malicious activity.
Without more information, it would be hard to figure out which is the correct answer.
Your friend should check the file and folder permissions to ensure they are not read-only. After that, they can see if there is a .tmp version which can be copied and renamed that might be more up-to-date. If they have Windows incremental backups they can also right click on the folder and see if there’s a better version they can roll back to. Other options depend on whether they are using Epic or Steam, and whether or not they had the launcher’s cloud sync enabled.