This sounds bizarre.
I would advise your nephew to call her up and tell her he is getting a different shadchan to arrange it. Then, he should ask her outright if she knows anything she thinks he should know. He should tell her it is his decision, and he is going forward unless she gives him the information and he decides not to.
The monkey in the wrench here, though, is why you are so involved on behalf of your nephew. Something seems odd.
My brother was once looking into a shidduch, and my aunt told him not to. Then, she refused to tell him why, and insisted he not go out with that girl. My brother said, “it is my decision, I cannot let you make it for me.” So she told him. And he didn’t go out with her.