You're right, the cable I described comes with the Cruise Commander M kit. I'm working on a couple of these and got the cables mixed up. The odd thing is, you have no use for this cable with the normal installation of the CC-M, so I'm not sure why it's in the package! You use the 4-pin cable from the old DCDR for the CC-M, so I have a lot of these cables laying around.
He has no cables with the Lionel part, so he'll have to fabricate those for this option. If he uses a standard RS4 motherboard, he'll be faced with the came cable dilemma. That was actually where I was going, I got sidetracked by the stuff sitting on my workbench.