Well, it looks a lot like an armored scale insect if it is scale. It would really help if the genus or species of the host cactus is known, since many types of scale insects specialise in a particular plant. With the identity of the cactus, you could possibly narrow it down to the species of scale insect, but more importantly, you would be able to find out all the pests that that plant can have. There is an article here on Garden Web in the FAQs section that talks about how to best remove scale infestations. I hope this can be of some help to you.

It is difficult to assign a precise name due to hybridization and natural variability within this cactus group, but "Cereus peruvianus" is a common name for them, and not worse than anything else. It is a fruit, it shoult turn red when ripe.

It's not easy to id which Pitaya variety will bear edible fruit. All cactus species will flower and bear fruit at some point in their lifetime, same with Pitaya. However, some Pitaya do not produce edible fruit (i.e. too small, too thorny, etc.) but they all should send out these spectacular flowers which bloom late at night. Like all cactus species, Pitaya thrives in dry, hot and poor soil condition. If there is too much water or fertilizer, they just get massive (like the one in your customer's backyard)and climb higher and higher without producing any flowers or fruits. Here in Hawaii, in dry locations on the island, local farmers grow Pitaya dragon fruits in cinder gravel and let them climb on lava rock walls. Sometimes hand pollination of the flower is required to make the plant bear fruit. I have a red, yellow, and white variety growing very well in my garden (yes, in plain cinder soil) and they had produced edible fruits. You can just cut a mature piece of Pitaya about 1 foot length, stick it in a pot filled with sandy soil, water sparingly and it will root within 2-3 weeks. Or you can find a larger piece on your friend's plant that already has clump of aerial roots. Pitaya love send out roots to attach themselves onto rocks or tree branches or whatever they're crawling over! Good luck. Aloha!

It could be in the pachycereus family. Also try Cleistocactus-jujuyensis