The only way to unlink your friend's XBL ID is to disable Cross-Save. You cannot unlink an account while Cross-Save is active. Please note, as the other reply mentions, your current save data is your Playstation save data. If you deactivate Cross-Save, your Playstation save data will only be available when playing on the Playstation console. Cross-Save does not move or merge your save data in any way with the other platforms you play on; it simply allows you to load one platform's save data on the other linked platforms. Even though you've been playing on Steam, you've been using your Playstation save data, and without Cross-Save, you won't have access to that save data on Steam.