Suggest Treatment For Stiffness In Knee Joint

I have a very dear friend who is a quad for many years now and has been treating a pressure sore. In the meantime, he has recently discovered he has one knee that no longer bends. He has been to 2 otherpaedic doctors who have basically said there's nothing they can do. I'm very upset about this and I'm desparate to find a doctor who can help him. Can you give me any suggestions?