This is not going to be easy. Perhaps post in the Android thread, there a lot of people running custom roms in that thread. Fortunately, if he has Cyanogen, it's probably rooted already. The problem is that the base operating system nolonger exists. The original kernel and rom have been wiped out.
http://forum.calgarypuck.com/showthread.php?t=89510
If the previous user installed the recovery menu, you have to figure out how to get there. On my Captivate, it has a Galaxy S Vibrant (I9000) kernel on it so it thinks it's a Vibrant so I press volume up, power, and home at the same time and hold them until it reboots into recovery menu. Yours may be that or both volume and power at the same time.
If you can get into recovery menu, you will be able to load new kernels and roms via choose zip to install.
If you really want the stock one, it will be tougher to find, you need the original Samsung kernel and the Rogers rom for the SGH-I896 Captivate. I'm not sure if you can even find that since most people on the web have the American I897 one.
I suggest trying a different kernel and rom like GALnet MIUI
http://galnetmiui.co.uk/landing/stable-builds/