Hi, I would say first of all, is your student getting the extra 25% time allocation they’re entitled to?
French is the most difficult language for dyslexics because of the phonetics, but they can be successful. I take it your student has done some practise essays. I would suggest that you or someone else records them so the student can listen to the recording and try to learn standard sentences that way eg je suis alle en France les vacances dernieres (can’t get accents on here!) This should work for writing and speaking. Key phrases like these can always be adapted to fit the question (holidays, free time activities etc). Remember that there is some margin for error allowed and the language only needs to be comprehensible to a sympathetic native speaker for a high mark, so spelling does not have to be perfect. Make sure your student’s vocabulary is secure for the listening and reading papers. Good luck.