The server creates a player data file for every player on the server (in server dir/world/playerdata). These files contain data such as the position, dimension, inventory, etc. of a player. The filename is the player's UUID.
You can edit the player's data file using NBTExplorer (which is a free piece of software that can read and modify NBT data). To do this:
- Stop the server
- Make a backup of the player's data file!
- Open the data file in NBTExplorer (not the backup, but the file in the playerdata dir)
- Set the Dimension tag to "minecraft:the_overworld"
- Set the Pos tag values to where you want to move the player to
- Save the changes
- Start the server