You mean the connector (from Xbox to TV) that you get in the box? If so, then yes you take that out and replace it with the HD lead.
That was assuming he had a HDMI post like mine... as you can see I've only just bought one myself :rasp:Wha?? No.
The connector with TV/HDTV is a Component AV cable. IF your XBox has an HDMI port, then it'll be separate.