I have inherited an unknown variety of cactus from a friend who moved. It was damaged during it's ride to my home (it is 6' tall) I don't know if you can see - but one spire actually was broken. It seemed to recover for a week or two but now the smaller spire is shriveling and turning black. I don't know what to do to help it. Any assistance would be greatly appreciated. I will attempt to provide .jpeg here.
Ok - I got an email indicating that this is a Cereus hildmannianus. And the damage looks like it stopped and I it appears to have been caused by cold temperatures. It’s hard to tell for sure, but once the damaged part turns a tan color, you can break it off and the plant will sprout arms from that point. Thanks to G.S. http://www.desert-tropicals.com/Plants/Cactaceae/Cereus_peruvianus.html
Your photo shows your cactus (Cereus) growing in the home. This is a big problem because the plant will not do well in the house. It needs much more light and some hours of sunshine during the day. I would slice off the damaged arm where it joins the larger stem. You could root this and start a new plant but it will still be a mess because of the dried up portion of the upper stem.
