How big is your tank?
What size are the discus you added?
The discus that died, did it show any signs prior to it's death? Like hiding in a corner or dark color?
I also agree that the discus did not die because it was stock in the foliage. I would have to think it died and secluded itself from others, which happened to be in the foliage. I would have to check the water parameters.
If you have juvenile discus, then more frequent water changes are required, since they need to be feed frequently. Otherwise, they will do well with regular weekly WCs. I have adult and sub-adult discus and are doing well, which are even breeding, in my tank with regular WCs every 4-6 days.