I'd go by the nature, rather than severity, of the dyslexia. If you know how the person processes things then you also know how to teach them. If there is difficulty processing space (if you mix up p q b d) or sequencing (c then a then t for cat) these are two different things, though they could exist together. The piano offers sound and touch. Dyslexia means an inability to read, but it doesn't tell us what is behind that inability. The word is used for many things, and it is misused.