-This species is not one of them.
-The kinds of cancer involved do not include brain and thyroid cancer.
-Even the kinds of cancer linked to some Euphorbia species appear to need fairly specific circumstances (infection by a particular common virus, repeated exposure to the Euphorbia sap over a long period of time, living at low altitudes, malaria, and eating a lot of cured fish and meats.

So no, there is no reason to think that Euphorbia milii gave your reader’s mother cancer. (And even if they had, there’s nothing to be gained from destroying a sick woman’s plant collection, unless you want to make her feel worse.)

They also do not cause financial catastrophe, they don’t kill or sicken whole families, they don’t contain evil spirits (or any other kind of spirits, for that matter), and they don’t grow if you stop watering them. Your commenters who say otherwise clearly have no idea how widespread this plant actually is. Believe me, if every family that owned a Euphorbia milii died, sickened, or were financially ruined, you wouldn’t be able to find any for sale.