We use the Pocket version of Minecraft on PC.
Is it possible to switch the Java version with his account or do you have to buy one again?
At that time there was the Windows 10 Edition free on the official Minecraft page to the Java Edition.
If you have not bought through minecraft.net, but through the Microsoft Store, you have to buy the Java Edition (https://minecraft.net/de-de/store/).
You can also buy in the media market or so Minecraft and redeem the code (https://minecraft.net/de-de/redeem/).
But if you bought Minecraft at minecraft.net then you should be able to log in to the Minecraft launcher with your Minecraft account.
Note: The action in which one has received the Windows 10 Edition for free has been over for some time.
Have a good time
