I'm kinda torn on this one. On the one hand, this is definitely negligence if the doctor didn't tie both tubes. In fact, he only tied the one that was defective in the first place. If he had tied the working one and left the defective one alone, I think that would've make it more complicated. But even then, he should've done both. Moo was left with the false belief that she was sterilized, when in fact she wasn't. That's not okay.
On the other hand, Moo did have options other than keeping the 4th baybee. She chose to continue the pignancy and did not opt for adoption, so she chose to take responsibility for the chyld. In that case, I think it mitigates the doctor's liability for future health care expenses.
I can certainly sympathize being stuck with a baybee that shouldn't have happened via incomplete sterilization, but she did have other options. This is a tough one.
Apparently their local law says a doctor cannot be held responsible for failure of the sterilization, and I'm not sure that is a bad thing. Sure it sucks for having an unwanted pignancy, but, otherwise if that law is disregarded, what is to stop future moos from having a defective baybee and trying to sue for their doctor/insurance/etc. to pay for lifetime care?