Witches can coexist(in the "nicest" sense of the word) with regular humans only if a) their abilities are subtle and nearly imperceptible so they are just slightly better than normal humans (doesn't seem to be the case in this universe), b) has very strict conditions on the use of their powers (also doesn't seem to be the case, as the Prime Bitch here seems to have violated their worst taboo but is still alive, and in possession of much of her powers), or c) there is a way to severely neuter their abilities, like the Tranquil in the Dragon Age universe. Otherwise the power imbalance is too great -- the world would then revolve mainly around witches and their whims.
So if Prime Bitch is really the last witch in this generation, then off her (she's violated plenty of normal human rules/laws/mores AND witches' worst taboo already) and find a way to detect and neutralize (one way or another) future witches.