HDTV Thru Coax?

[flex411]

Hello my friend has a sony plasma HDTV, charter cable with the high definition package. His cable box about 100 feet from his tv. An employee at tweeter suggested he use rg6 coax as the cable to run from the box to the tv. The signal is not good at all. And it gets worse when the light dimmers are on. He has tried putting new ends on and running new cables.
My suggestion was to move the cable box close enough to get an rgb cable from monster to it.
My question is, Although it is possible to use coax as rgb cable, is it reccomended? And when he watches HDTV and it kicks in to 16:9 is that signal really a High definition signal?

[edDV]

Best to get the cable box near the HDTV and keep the RGB cables to less than 12 feet if possible. If this can't be done, very expensive cables will need to be used. Good long RGB cables can be purchased here.

http://www.ramelectronics.net/html/hdtv-cables.html#com
http://www.bettercables.com/

The RG6 cable coming into the cable box from the cable company can be longer with less loss since the box has internal equalization.

RGB cables (actual signal is Y, Pb, Pr) are three equal length coax.

The TV should be looking for 1080i (usually) or 720p for HDTV

1080i, 720p or 480p (EDTV) can have 16x9. The TV and/or cablebox should indicate which standard is being received.