It is possible to use the Orton-Gillingham (OG) approach with an older student with dyslexia. An older student has the advantage of more schooling and exposure to reading and spelling rules. The problems is that some of them have stuck and some of them have not. The OG method allows for the systematic review of the phonological process to see areas of need.
I love to help older students because it is like being a detective in a real world mystery (where no death has occurred). The OG method give me a secret treasure map of where to find the treasures needed at every turn of reading and spelling. The students come with some negative experiences related to reading and writing. I have always felt that half of my job is keeping them positive, while the other half is teaching the actual skills they need. We work together to gain mastery over the weak area. The students are always amazed at some of the things that they never learned in elementary, middle school or even high school. Sometimes it is something they have never learned, and sometimes it is looking at it from a new perspective.
If you would like me to work with your older student, contact me.