As a member of the Veterinary community i have to say that you guys are awfully quick to judge what is going on without knowing the profession or the case. While i'm sure it can happen, it is highly unlikely that either clinic is running up the bill or charging you more because they know you will "do anything". Science, let alone medicine, is not black and white. It's even more difficult when you patient can not tell you what they are experiencing or going through. It can be very difficult to come up with a definitive diagnosis in some cases, especially in an older dog. That said, i would have expected some sort of partial resolution to the symptoms at this point. Based on what you are describing, i would be honest with the client and say that you may have reached the time to evaluate her quality of life and how much you are willing to spend to MAYBE prolong her life. I'm sure the clinic has ruled out intestinal parasites and Parvo, these are unlikely causes of diarrhea in a geriatric dog. I wish you the best but if you want an honest opinion....i think it's time to start thinking about quality of life (based on your description of things).
Cschelk2 DVM