It may be that he could be sued, or at the very least violate his own oaths or otherwise be held responsible, if he didn't provide a service.
True, though it's a bit of a messy issue. Legally and ethically the doctor can't dictate what's going to be in the patient's best interest. It's ultimately up to the patient to decide what to do with his/her body. This falls under medical ethics: the patient has the right to refuse or choose their treatment. Likewise, this is combined with beneficience, that the doctor must act in the best interests of the patient. If the patient had said she was quite able to take care of the child (at that point in time there was no decision about keeping all eight of the embryos), then there would be no reason for the doctor to refuse to do the procedure.
The doctor would have taken steps to cover his own backside, such as statements from the patient indicating full responsibility for this action.
Doctors say no all the time to patients wanting certain stuff done that they shouldn't have.
Give me an example of this. The only time this would happen is if the procedure would do harm to the patient, and thus the doctor's legal obligation is to tell the patient all information they'll need to know to make their own decision.
Sure it might have been irresponsible of him, but doctors are human too, they can't be expected to know everything.