Did you not pay attention to the movie? Everything that happens makes sense to her character. She doesn't just immediately search through his house for clues when the news comes out she gets bits and pieces of information and tidbits that get into her head to the point where she needs to know for sure.
That scene was weird. She is using a normal voice to invite Abe to eavesdrop the other people, who can just as easily hear her. In real life one would signal something with a hand gesture or scoop over to his side to deliver the message. It was like the other people were not there. Sloppy.
There were a few other scenes where Stones acting was not believable. While she is playing piano with the teacher sitting next to her, she is moving her body in a way no piano player ever does. When she is checking her phone to see if Abe has messaged her happy birthday, she over acts with her facial expression. Why would she be so disappointed in the middle of the dinner? The evening is not over yet and Abe could still message her later. The disappointment should have been played in a more subdued way.
I think Allen had Jill alerting Abe to the conversation with the woman in the diner in order to show that it was Jill who started this whole thing---when Abe heard the woman's story, he saw it as a direct way of doing something good--in his mind--for him, killing a dishonest judge who made life miserable for people (he did research on the judge and found he was corrupt but un-touchable because he had the backing of powerful people)was a positive thing. So he kills the judge, he feels alive for the first time in years. Also, Allen wrote Jill as being kinda ratty to her boyfriend. There's a reason for this. I don't think Jill was this great person out to see that justice was done. On the surface, yes--she was outraged by Abe killing the judge--but she comes off as this shallow, pushy, shrill person---who walks off the picture happy---where alot of people on this post would've like to see her going down the elevator shaft. I think this is Allen's sick joke.
For thematic reasons, I see why the act would satisfy Allen... but I don't think that her character motivation to do it in the first place rings true...
"Your mother puts license plates in your underwear? How do you sit?!"