I think the reason nobody has answered this is that it isn't an easy question to answer.
Let me try and get this straight. First, he is being fed by Comcast and has a single cable running through splitters into several rooms. He then is feeding each signal into either a cable box or cablecard. He may or may not have a direct feed into the digital TVs to bypass the cable card/box. Is that about right.
If so, one problem is going to be finding space in the Comcast used bandwidth to insert your signal. Comcast tends to use the VHF space and some of the other lower cable frequencies for analog feed. These days they are jamming stuff into every spare bit of bandwidth in order to provide more and more HD material. It is even difficult to figure out what space is being used, because Comcast is multiplexing and remapping channels in the digital space.
If he is willing to run a second line, there are lots of options, both RF and video.
