She doesn’t appear to think too deeply, as she’s come out with a multitude of lies which were easy to disprove.
However, you don’t get ill or die straight away; from what I’ve read on Google you have food poisoning type symptoms after so many hours then typically appear to get better for a while, before suffering organ failure (3-6 days after eating them). So if she’d done her research (and there is still a missing phone) it’s possible she thought they might have had further meals together before becoming seriously ill.
Also, evidence of DCM poisoning is apparently only detectable for two days after eating DCM, so there was a chance they wouldn’t have been able to detect the cause by the time they got to hospital or suspected it. A slightly different scenario, and it might well have ended differently.
I don’t know how close the couples lived or how many hospitals there are in the area, but weren’t there multiple hospitals involved? So they could easily have ended up at different hospitals with no communication between them.
Don’t forget that SP was supposed to attend too; she sent a message probably in the hope that he would change his mind after he backed out, so she may well have hoped that he’d turn up on the day after sending that.