To be fair, vets sometimes send pets home with a catheter still in their leg, if they're expected to be coming back soon and needing it again -- saves the trouble of finding a vein again. So it's not totally insane (though pretty close) for someone fairly dimwitted not to realize that when being discharged from the hospital with no instructions to return, that thing definitely needs to come out first. HOWEVER, it is totally insane to go to a lawyer for help, when you finally realize this can't be right. Had she already called the hospital, before she came to you? Perhaps whoever answered the phone at the hospital went off on some rant about "I can't believe those idiots left that in you, they should have known to check, yadda, yadda", thus giving Dummy the impression that she had been the victim of serious medical malpractice.
No, you would definitely have to be an imbicile, moron or idiot to ASSUME that the device has to stay in your arm, rather than simply ask someone directly.
Nope, that was part of my initial intake with her as well. She had had no contact with the hospital since her release with "the thing" two days earlier. She wanted to sue and had the impression that the longer it was in there, the more she could recoup.