I cant tell what is going on the pic.
If that lighter color in the entire center of the coral has all rtn'd........there is no reason to wait
whack it up into as many frags as you can get. cut well into healthy tissue, and seal all open wounds with a thin layer of superglue or epoxy. You have lots of base left there it appears, after you cut all the frags off seal all open wounds left on the base with a thin layer as well.
as far as dipping after an rtn event....... pests dont work that fast, and all dipping does is add unecessary stress. Unless this is a recent addition, the rtn even was caused by soemthing in yoru water parameters, and or possibly not enough flow and the coral finally just gave way in the center, which can happen
how long have you had the piece??