What is the ruling on attending lectures given by a female doctor who denied the Sunnah and the Hadith, knowing that she will not repeat that, and there is potential harm from not attending?

Those who speak falsehood must be denounced. If they cease, then all is well; otherwise, it becomes obligatory to leave the place. Responding to deniers of the Sunnah is a form of jihad with the tongue. It is permissible to attend future lectures, but if the lecturer speaks falsehood, you must denounce it. What is forbidden by Islamic law is sitting while they are speaking falsehood. If they stop, then there is no harm in sitting, as Allah the Almighty says: "So turn away from them until they engage in another discourse."