Apart from fairly radical views that would prohibit any human use of any animal, I see nothing wrong with the basic idea of having dogs providing diagnostic assistance. We know that dogs can be really good at sniffing out explosives and bed bugs, for example, as we already use them for such tasks. If they turn out to be also especially good for medical diagnosis or troubleshooting, it seems like a reasonable thing to have dogs do for us. Obviously, the same rules about humane treatment for the dogs applies in these cases as for any others, and we would also want to have strong support from medical studies to confirm that the dogs really were helpful and reliable for these tasks.
Even if we have only some reason to think that dogs are good at this, then they could be regarded as potentially good indicators of some problem. So if a cancer-sniffing dog reacted in such a way as to indicate that I had cancer, I think I would be well advised to go and get a check-up, even if there was not yet the level of medical evidence we would want or need to adopt cancer-sniffing dogs as a regular part of diagnosis.
I fails to see any serious ethical problem here, again, as long as the dogs are well-treated. people and dogs have gotten along well together for a long time. A little shared working together doesn't strike me as anything wrong at all. But again, this is to set aside the objection that any human use of dogs is wrong. I don't agree (obviously), but this is not the place to take on that sort of objection.