A person with dyslexia will find it hard to read accurately and fluently. This means that they will be very slow when you give them something read. However, they will not have a problem understanding the text when you read it out them (unless they also have another difficulty which is not uncommon).
A person with dyslexia will seem to understand you perfectly when you explain something but will find it hard to follow the same content in writing. Often a person with dyslexia will also seem to 'mangle' difficult words, especially when you ask them to speak suddenly.
Because of their short term memory difficulties, dyslexic people will also struggle with following instructions when given in rapid order.
Each person with dyslexia will have individual needs. But there are a few things you can keep in mind.