SSDI I don't know if you already filed but here are some guidelines I posted to help people who are filing. You may want to read 404. of the Social Security Act this is what they use.
I always found it helpful to include case law as well as the sections. You have to have medical proof that your friend cannot sit, stand , walk for long periods of time, also have proof that your friend cannot stay on task for very long. Make sure your friend has both physical as well as mental proof. Good luck.

http://neurotalk.psychcentral.com/thread115518.html