ADHD children are often quite gifted; schoolwork is just not the best avenue for them to show it. The knowledge is there, but they find schoolwork to be tedious, and the distractions of the classroom are a bit much for them.
My ADHD son is extremely intelligent, but struggles with "schoolwork." He knows his phonics sounds, but struggles to read. Writing is a struggle for him as well. We discovered that the problem was not dyslexia, but rather a problem with his visual tracking and focus. If the testing for dyslexia is negative (and perhaps even if it is not), your friend may want to consider vision therapy. We have started my son on some basic exercises, and are already seeing a huge difference in his reading and writing, as well as his ADHD (we do not medicate).
Hope that helps.