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!