Adding to F4E, in some future if you need its assistance or needed to temporarily uninstall norton security for some reason, windows defender can give protection to a small extend. I wont expect that to happen.
Since windows defender is a built-in feature, you have no way to uninstall it(assuming you have win 7 or above)
The only work around to limit windows defender’s functionality apart from disabling (managed by norton) is to
Open Windows Defender from Control Panel -> go to Tools on the top menu -> click on Options -> click on Administrator on the left-hand pane -> uncheck the box for “Use this program” -> click the Save.
To make sure that it never comes back, you can also open up the Services panel through Control Panel, or by typing services.msc into run . Find Windows Defender in the list and double-click on it…
And then you can change Startup type to Disabled. I personally donot recommend this.
His purpose of getting rid of Windows Defender is to make more room on the C drive. Disabling it won't give him more room... I think that is why he wanted to remove the program.