I thought what she said was that SS told her Amy was going out Wednesday (not Wednesday night), so that when he later said Amy had been home sick in bed all day, that struck her as strange because it was inconsistent with SS's earlier statement that she was going out Tuesday (daytime). MOOsnipped
It think the "Amy is going out tonight" comment was part of an everyday conversation the two had when they were in person. That's how I originally took it. I'd like to see a friend or associate's of Amy's speak up and confirm s/he had plans with Amy that were canceled Wednesday night. I would think that if Amy were indeed going out (which seems to fit her social reputation), she would go out with someone and not alone. So, did that person receive a phone call or text from Amy to cancel or was the person stood up?
We only know this info because NG said the conversation happened. It's possible Amy had no plans to go out and NG concocted the story. At this point, I'm still believing NG but the schedule of her week is nagging at me.
I didn't think so before, but I now think SS saying Amy was in bed all day Wednesday is a clue, either for NG to be concerned or later for LE to figure out the happenings of the day. If he says Amy was in bed all day, maybe that's a clue to LE that any withdrawal from the bank on Wednesday was NOT by Amy. ???
I do not suspect SS and NG were having an affair.
JMO.
Edited to add: like you, I took it as ordinary chit chat.