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.
Late allowance in-depth
Setting a late deadline for a student works the same way as moving the normal deadline – but only for that one student. This means that if you're allowing a student to give feedback late, but that student hasn't handed in, the student won't be able to give feedback. In that case you will want to also allow the student to submit late. Once they've submitted late, they will then be able to give feedback late as well.
In the case where you've allowed all students to give feedback regardless of whether they've submitted, the student will be able to give feedback as soon as you set their late feedback deadline.