"The student loans a book to their friend."
Translation:학생은 친구에게 책을 빌려줍니다.
It's correct, it's just that Duo only has a certain number of correct answers listed. If you flag it, the translation will eventually get added.
(It may take a while. I stopped actively using Duo around 12 months ago and every now and then I still get email alerts to notify me that my alternate translations have been added to the course.)