Linux

Keep in mind that there are 2 versions of Minecraft:
The Java edition: Runs on all Operating Systems
The Bedrock edition (Previously known as the Win10 edition): Runs on Phones, Consoles and Computers, but as old name implies, only runs on Windows.
This is the only version that supports cross-play with Consoles and Phones!

Although there are ways to run the Bedrock Edition on Linux, they're all unofficial and I don't know if they still work properly. You should check what version your cousin and his friends are currently using, as the Java version will not allow him to play together with his Console/Phone friends.