To show that an opinion has been formed 'in good faith' does not mean that authorising an abortion must be the 'right' course of action, simply that the doctor has not been dishonest or negligent in forming that opinion. What makes an abortion lawful is the doctor's opinion that there are lawful grounds for the procedure, rather than the fact that those grounds exist.
So, for example, if two doctors believe in good faith that abortion carries less risk to a woman's physical or mental health than carrying the pregnancy to term, this makes the abortion legal – even if, in the eventuality, it would have been safer to carry the pregnancy to term (for example, if the abortion resulted in death or injury). Similarly, if a woman states that she cannot afford to continue the pregnancy, the doctor is not obliged to check that she really is lacking in funds.