I have a SGX with a SCD2 attachment. This gives me two A/V outputs: one on the SGX and one on the CD player. I'm not familiar with the briefcase set-up but I assume that your friend has the same options. With my set-up, I've also had problems using the jack on my CD player. The video is OK but the audio is very quiet and scratchy. As soon as I switched to the output jack on my SGX everything cleared up and I've used that one ever since.
If that doesn't work I'd say your friend's cable is on the fritz. The video plug on my original Duo A/V cable got faulty and broke a couple years ago. I cut off the yellow plug, opened up the wire and spliced on a new plug from an extra cord I had. It's been working fine ever since. You can also order one at TZD or japangamestock.
I also have a Supergrafx question of my own:
Can an RAU-30 adapter successfully connect a SuperGrfax to a SCDRom2 drive? I'm guessing probably not but, if it does work, then my RAU is broken.
I realize this is completely stupid and unnecessary--the SCD drive has no problem fitting directly on a SGX. I ask because I picked up a cheap, used, RAU-30 recently and would like to test it to see if it works. I don't have a briefcase unit to test it.
I purchased the RAU because I have a chronic fear of another CD-drive failing on me and wanted the option of buying a cheap briefcase unit in case anything terrible ever happens to my SCD attachment.
(Sooooo...when are we going to see that technical advice forum!)