What are its growing conditions (soil, light, humidity, air circulation, feeding/watering schedule)? Are those other capensis plants on the side of the pot?
The pics are a bit fuzzy, but based on these and your description, it seems like classic leaf rot to me (center plant). Immediately cut off the black/brown portions and apply sulfur powder to the cut. You may also want to lightly spray some fungicide (Captan) on and around the plant if you have any. Let the soil dry out a bit, place the plant in a bright spot with good air circulation, keep the leaves from touching the soil or getting wet, and pray.
The other possibility is that the plant is responding to cold (<50F), but only you would know whether this is the case.
If the center of the plant is still green, it may be salvageable--sometimes the root system can form a new plant even if all of the topgrowth is dead, so I'd keep the plant for a while even if it ends up "dying" (just isolate it from other plants).
Hope it makes it.