Should a committed female university student, who is corresponding with a male colleague via email (regular messages and photos), continue to respond to him for fear of insulting him or affecting his studies, or should she stop corresponding with him for fear of things developing without her family's knowledge?

You did well not to respond to him, as it is not permissible for you to correspond with a man who is a stranger (non-mahram), because it could lead to what is forbidden (haram). If he desires you, let him propose to you through your family. Beware of such relationships, for they are a gradual enticement (istidraj), and adhere to keeping away from men who are strangers (non-mahram). If there is someone you desire and whose religiosity and character you approve of, let him propose to you through the lawful (Sharia-compliant) channels.