Before writing, the author should engage brain. This mangled grammar shows ignorance.
I love dumb asses like you who read things and look for typos and then if there is one somehow try to discredit the message. I read articles by syndicated columnists and on sites such as the Wa Post, NY Times, WSJ etc and see typos all the time. No one comes on the air and says Charles Krauthammer can not be believe because he used “to” where he should have used “too”. In addition, while I would agree “frightened” fits better I actually am not sure that you are in anyway correct in your assertion that the use of the word “frightful” is incorrect. It is not incorrect to say that someone or something can become frigthful.
I retract the last part of the prior message because although someone or something can become frightful that would be the adjective describing them, it would need a verb as in frightened as you asserted to fit into the sentence in question. Nevertheless, your comment is quite frivolous.