Is it permissible for the kaafir to enter the mosque to listen to a lecture?

Yes, this is permissible, if we are confident that this
kaafir will not disrespect the mosque, because he is entering for a reason,
and not to harm or damage the mosque. Similarly it is permissible for a
kaafir to enter and stay in the mosque to repair something in the mosque,
because this is in the interests of the mosque, so it is also permissible
for him to enter to listen to lectures which may be the cause of his being
guided. The Prophet (peace and blessings of Allaah be upon him) tied up
Thumaamah ibn Athaal in the mosque [when he was a prisoner of war, before he
became Muslim]. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).