I'm going to refrain from making a new bet for the moment, because I really don't know who to bet on next.
I'd say just allow it, the goal of this game is accurately predicting the flow of the story and at this point the question as to whether the demons will be able to stay alive long enough for the summon to expire is not exactly clear cut.
Also, just to check, if we were allowed to bet on the fiends and the summon expires before they are killed it doesn't count as a death, right? How about the silicon elemental? (I think it seizes to exist once the spell expires, instead of being sent back to wherever the fiends came from, though I could be wrong about this)