Maybe too wet in too big a pot for one stem. I dont think its savable.

If you cut off what looks like a rotted top piece, it should put out new growth.

does the pot have drainage holes?

Yes the pot has a good drainage hole. Based on what I've seen online and when I dug it up to check out the roots, I think I want to sanitize everything and start over in a smaller pot since the original reasoning why he put it in such a large container was because there was supposed to be several other plants sprouting that never did.  Can this be rerooted from the lower section with leaves, or a middle section further away from both sources of concern? Will the middle produce roots and grow back to normal in brand new soil and a smaller sanitized pot? 

Most of the roots were brown like every other plant that I've had die on me,  but there is one noticeably white one still. Is it better to cut everything except the one root and not rotted plant and sanitize everything, or cut somewhere in the middle and completely regrow everything?

Also, should I get a special light for the winter and wet season since I'm pretty sure the rain is what triggered this to begin with?

When did it last have water?

I grow mine near south and west facing windows, in NY. I use unglazed clay pots and pro-mix potting mix with extra perlite. They get thin, long, and weak when they don’t have enough light, they bloom less, and their bloom and leaf color is faded… you’ll know if they need more (or artificial) light.

I usually water it every 1-2 weeks depending on how the soil feels through the big  drainage hole, but I let it go 3 weeks this time since it got over watered in the rain and mushy. I watered it last night after pulling it up to check out what's gong on with the roots.

I think it might be possible to root a middle section. If you want to try it, I would cut a piece about 3” long, and dry both weeping ends with a papertowel, then leave the cutting to dry and callous over for at least a few days. Then pot the cutting, making sure you plant the bottom end down.

Soil looks too organic, that may be a reason it doesn't drain well enough. IMO, it is also in big pot (all that soil stays too wet for too long). Mine stayed outside whole summer long & was rained on. But it is potted in a mix that drains well. I have few, most in plastic pots but have 1 in terracotta pot. They seemd to do equally well. I don't think that top is salvageable; I would cut it & hope rest is not rotting. JMO!

At some point you allowed the roots to dehydrate too severely whether from a poor soil mix or from under watering. Then they rotted after they were watered while dead. The top usually is the first part to fold then right to the bottom.

I would cut that part off and re pot into a very porous mix that allows you to water frequently without fear of over watering. These plants don't like to dry out like cactus and in fact require a mix that stays evenly moist and lots of light.

The little guy turning yellow on the side is a perfect example of far too little hydration.

Under water these plants and they die. Then the roots don't take up moisture or oxygen to stay healthy allowing them to die further.

Next time, just use a better mix and water as soon as the mix feels dry within a week.

By the way, you have far too many huge chunks of bark in the mix allowing the finer particles to accumulate where the root are keeping that area too wet or too dry and other areas too dry.

Mike