Playerfiles contain any player related data on a server. Among others these are inventories, achievements, location etc.
Sometimes it's required to delete someones Playerfile, usually if a player got bugged or if someone did some expermiments with a player like teleporting him far far away into the sky.
Deleting someone's playerfile causes any data related to the player to be reset.
1. Find out the UUID of the player
A UUID is a longer string which consists of several random letters and numbers. This UUID is used to identify each player on your server.
If you already know the UUID of the player who's playerdata you want to delete, you can skip this step.
Start your server and go to the Console page.
Now let your friend try to join the server. A message will appear on the console that a player tried to join the server. This message contains the users username and the users UUID.
Note down the UUID of the player.
2. Delete the playerfile
Now stop your server and go to the worlds page.
Click the files button on your main world.
Navigate into the directory called "playerdata".
This folder now contains all playerdata. Every player on your server has their own file. Find the file which is named after the UUID of the player who's player file you want to delete.
The playerfile is now reset. Start your server again and let your friend try to join again.