Assesments

In this article under "Understanding 1", Carol Tomlinson writes that she had students in her class who failed tests but showed other evidence of learning. My question is, how would a teacher go about grading these students?
I had a student once who showed up late to class, didn't do his homework, but when I would give class work at the end of class he would learn the material quickly and help others with the assignment. It was obvious to me that math was not hard for him to learn, he just needed to practice it for a few minutes. However, he was not able to retain the material long enough for the test day. On test days he had forgotten the material and would do poorly. If he would have done his homework and studied a little, he could have passed with flying colors. How do you asses a student like him?