If you want to allow a student to submit late feedback go to Feedback tab on the assignment page. Scrolling to the bottom of the page, there is a list of students that did not provide feedback. You can either allow all students to submit late feedback or allow individual students to give feedback.
If you need to allow a student to change or finish giving feedback, find the students name on the Feedback tab of the assignment page. Click the blue drop down arrow then click on allow student to change their feedback.