Pamela,
I think you are right, Pamela. Knowing about a students difficulty -- IF they are comfortable in sharing -- can significantly improve your interaction with that student. It is not always possible (since not all students are comfortable with disclosing), but when you are able to establish a relationship of trust and get that information, it certainly helps.
Just a clarification, though. A head injury (as you describe it here) is not the same as a psychological disability. A closed head trauma (the kind received from a car wreck) can cause cognitive difficulties, which is what the student was probably describing to you.
Dr. Jane Jarrow