First off, she might have learned to ask the question that would have prevented a mildly-awkward scene in the first place: "Is that actually Fred Willis or someone who looks like him?" It seems to me that a normal people's reaction to his dinner companion not being Mrs Willis would be to speculate that the stranger bears a certain resemblence to someone she knows. Since Elly has convinced herself that all men must cheat given the opportunity, she can't help but assume the worst of the men she knows. In her mind, the possibility that that could be Fred committing adultery means that no other option can be reasonably entertained.
Another option that cannot be allowed to be considered is the concept that just maybe, attractive women aren't all conspiring to seduce husbands so that they can laugh at the housewives that their good looks force them to want to make miserable. Simply put, the unknown woman was tried and convicted because Elly never outgrew the middle school assumption that the pretty girls wanted to steal away all the boys she was interested in because they hate homely girls and want them to suffer.
Not, of course, that Elly would be much use to June in such a situation. As we'll see, Elly has little desire to be in the presence of women who "allow" their husbands to cheat; the same impulse that led her to want to shun June like a leper kept her from associating with her 'weak' friend Anne for years on end. She's even less use as a scold because the instant her unshakable assumption was proven wrong, she just sauntered away without doing anything.
The most annoying thing of all is that Elly doesn't want to admit that she expects better of her friends than herself. She simply will not admit that her refusal to make it clear that John's nauseating habit of being visibly aroused when in the presence of the pretty is a problem is seen as licence to keep on having hormone attacks.
The interesting thing is that the man and his companion probably don't remember this little incident at all; Elly, on the other hand, probably still burns from the embarrassing time when some man and his floozy decided to trip her up and make her look like a fool.