So I gather that it depends on the type of film and the actual chemicals involved. I was wondering if the colour couplers were still present in the faded film if they could be restored by some form of reprocessing. I will pass this all on to my friend, I think the way to go will be to get a good scan done first before trying anything. If re bleaching does restore some colour then can scan again and photoshop will have a bit more to work with.
If my friend still wants a transparency then there are companies who will produce a slide from a digital file.