...how can I easily get him to determine which box (and firmware) he has?
Tuta - He would be very lucky to have the right firmware on the box used for HD service at that TV. To check firmware on the 5100/6200, go to the Diagnostics Menu by powering off the cable box and pressing OK/Select promptly afterward. Item d08 in the list is Code Status, a right arrow brings that screen up. My firmware shows 7.10, thus DVI is activated.
If he shows 5.04 or anything starting with less than 7, then he needs a box swap as TWC currently does things. Have him follow what Mikekask or I did - for me it worked better to go to the neighborhood TWC office and request active firewire output on my HD box. Don't take your box in.
CFC - it only matters that you pay for HD service, so you can check either 5100 or 6200. But beware - an active DVI port still won't function unless it gets a 'handshake' from a TV which is HDCP compliant. Search DVI-HDCP on the forum if this is a surprise.