How to transfer player data for a Minecraft server

You would find their playerdata file, in the server folder/playerdata, and change their latest file to their new UUID. You could also use a plugin or mod that allows you to access their inventory in game, and you could move the invenotory manually this way.

If your friend's old account is still active then they could put their belongings in a chest and log in to their new account and pick everything up. the only issue with this is the xp levels would still be on their old account.

hope this helped!