This information has been posted a little way down. There is several links to download that will help you. I don't want to repeat all of it here again.
Good Luck with the plant. Norma

This is not the best time to repot a cactus but in life or death situations we have no choice. Here is what I would suggest, find a terra cotta pot that is a little smaller than the plastic pot the plant is in now. Get a bag of cactus mix and mix it with pumice, one part cactus mix to four parts pumice. Plant the cactus in it after shaking off all the old soil from the roots. DO NOT WATER THE CACTUS FOR AT LEAST A WEEK AFTER PLANTING IN THE NEW SOIL. This allows the damaged roots to heal so that they are resistant to fungal attack.
You can find pumice at many animal feed stores as a product called Dry Stall. It is used in horse stalls to absorb moisture. Before you use it you should wash out the dust by putting some in a bucket and running water over the top until it runs clear. You want particles that are about 1/8" in size. If you can't find pumice look for scoria (crushed red or black volcanic rock) of the same size. Wash it also. Let the pumice or scoria dry out before you mix it with the cactus mix. All of the commercial cactus mixes I have seen contain way too much organic material and will retain too much water. That is why they must be mixed with some inorganic gritty material with large particle size. After you begin watering the cactus let the soil dry out almost completely between waterings. Dig down with your finger up to the first knuckle, if you feel moisture don't water. The general rule with cacti is "If in doubt, don't water". Good luck. Jack

"I thought it was to[o] dry when I saw the dirt pulling away from the plastic container"

This is the problem w/ soils containing so much peat -- the mix is pulling away from the container suggesting the rootball has hardened off & become too dry to re-wet.

Jack's given you the right advice, likely your best shot to save the plant. The pumice or similar material keeps the mix crumbly & fast draining w/out allowing it to beome too dry & hardened off like this. If you can't find pumice, try using perlite, will work much the same way.

You seem to know this is not a plant that wants direct sun; it is a jungle cactus, not from the desert, so it naturally grows among the branches of trees. It wants bright INDIRECT or DAPPLED light (under another plant, through the trees, behind a light curtain, etc.)

Btwn 2 or 3 threads here (as mentioned above) & one current over at Houseplants (abt 1/2 way down the page) there's tons of info. on the care of this plant. It's good that you've already been reading some of it. Good luck!

I agree with the post above that repotting now in a good quality cactus mix is a good idea. Adding the volcanic sand also enhances the trace nutrients in your mixture and will result in a healthier plant. I add volcanic sand to all of my planting mixes. The perlite will aid in drainage, but it adds no nutrient value to your mixture. I usually pick up the volcanic sand at Lowes or Home Depot and a little bit goes a long way, so it's well worth the cost of adding it to a ready-prepared mix. Cheryl

I would also like to suggest something since I haven't yet seen it posted. If another branch does fall off, ( which I hope it doesn't!) you can plant those stems into a new pot to attempt to root the cuttings for a new plant. I wouldn't try to take cuttings unless they fall off. but I thought I would just mention, It's not hard to take cuttings all you have to do is, if a large branch falls off, you can break that branch down into 2 to 3 segment cuttings, the more cuttings in a pot the fuller the plant will be, I use 3 stems per pot and the plant becomes fairly full. let the split up cuttings dry 24 hours to heal over and plant them into some soil, mist with a sprayer everyday until roots have formed, you will know when roots have formed when you tug LIGHTLY on the stem and it feels anchored down. It takes about 3-4 weeks this time of year to get roots for me. I hope this information helps, and good luck on your plant!

Thanks for all the help everyone. I'll give the plant some special TLC this weekend the weekly grind keeps me really busy. I'll keep you posted on our progress.