It's the thing. If he goes for the default build and has no MT insanity skill, he can only inflict it on her if she's equipping the Blood Chalice. And if he's going for the default, she has no reason to equip it -at all-. Assuming both default and both alternative, she still gets to go first and cheese him out - she only loses if he goes for the default and she goes for the alternative, which, given full equipment option knowledge, makes no logical sense. There are more guessing game outcomes where Cthulhu loses than wins.
(Of course, if he -does- have MT insanity - which I'll check out just now - this is all moot and I'm changing my vote.)
EDIT: Just checked. Nope, all ST, unless he gets it off some arcane alternative level build (and my sympathy for mix-matching Cthulhu level builds midseason is pretty much zilch).