First things first: What are all of the vital paramters (e.g. temp., dH, pH, NO3) in your friends tank? Knowing what the values are might help us help you better!

In my experience and through speaking with some of my friends who have been involved in the aquatic trade through the years, I would state that cardinal tetras generally have one of the higher mortality rates amongst all of the tetras. Capture and shipping probably accounts for a lot of the losses. Also, this fish is very hard to breed in captivity and thus, most of them are wild-caught in the Amazon Basin of South America during the dry season; meaning that they are subjected to a fair amount of water parameter shock upon arrival to the LFS and they are already somewhat weakened and skinny.

Also, though rare, the Oto's in that tank could be attacking the cardianl tetras. Some Oto's seem to prefer the slime coating on fish more so than algae and they will attack fish of similar size in order to get their fix of slime coating... determining if this was the cause would likely require hours of careful observation to determine if any such behavior was noted. Also, they do not always fair well with cichlids, so those blue rams could be attacking them too.

One thing I know for sure though is that cardinal tetras are generally more hardy at higher temperatures... say between 81F and 85F (similar temperatures to a discus tank). They also do best in fairly acidic water with a pH of 5~6 and slightly softer-to-mid hardness 2~6dH. They also like heavily planted tanks with some shadowy spots to hide and tend to do better with darker colored substrate.

-Jimbob