I’ve had success growing cactus from cuttings. I don’t see any reason why you couldn’t cut off the part above the damage, harden it off and then plant in cactus medium. With support until roots develop, it should be fine. Do a little research on the details. As it is it will not repair itself.

I agree with the above procedure regarding cutting the stem above the damage and planting it in a "cactus" medium (use a stile knife). Keep the medium only SLIGHTLY moist. In fact, if you are using a prepared soil mix, do not water it after planting. And speaking about planting DO NOT plant it deeply or it will ROT. Aim for only an inch or so deep. Of course it will be top heavy so you could use some big rocks or bricks to prop it up. It WILL eventually root. Some people brush rooting hormone on their cuttings, but it's not even necessary with this plant, which btw, is NOT A CACTUS, it's an African Euphorbia.

P.S., When you take you cutting WEAR GLOVES and cut it over newspaper as it will leak sap, AVOID GETTING THE SAP ON YOU SKIN AND EYES!! And I would not use the knife in the kitchen again!

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