My girl Jazz was diagnosed with Cushings about 4-5 years ago now and is still going strong. SHe was about 9 at diagnosis and was treated with Vetoryl (expensive but very effective). Her symptoms were kept well under control with it for 3 years or so until she started going Addisonian (opposite of Cushings).
She is now off her medication all together (which has amazed the vet) and ticking along nicely. I would say she is probably aging quicker than I would have expected her to but apart from that I am pleased to still have her here this many years after diagnosis
Hope your friends dog responds as well