"They" don't arrive at that decision. You do. (or your girlfriend) You (or she) will have to listen very carefully to the diagnosis, any possible treatments, the estimated prognosis (for whatever might be wrong) and any possible palliative or "hospice" care recommended if the diagnosis isn't good.
So these are the basics:
**What's wrong (the main health issue, not just the signs of natural aging)
**Is there anything can be done about it?
**Is there any hope for a decent prognosis with treatment?
**Is the treatment likely to be helpful, or debilitating?
To a dog "quality of life" is important. A dog can handle failing eyesight, increasing deafness, slowing-down, doing a lot of sleeping, not being able to go up or down stairs, not being able to jump in the car, and aging generally. But what a dog can't handle is not being able to eat or drink without vomiting, not being able to get off a bed, being in pain, urinating or defecating in the wrong places, not feeling like wagging that tail any more, being indifferent to loved ones or not wanting to be bothered doing anything any more. If things are like that and no medication is staving of these types of symptoms, it is better to make that appointment, spend a last day all together (time off work) and let them cross that Rainbow Bridge.
But if this is in any way treatable, or even if a "bad" diagnosis, if palliative care will help stave off symptoms and keep a fairly decent quality of life, then ride that out till the signs become clear that medication isn't working.
It's so important to be really tuned-in to a dog's needs at this stage of its life and to be very observant for sudden changes. Nursing care and observation is everything. And being ready to make that call if needed....the one that will call the duty vet out in the middle of a night to put her to sleep in the comfort of her home, if and when it comes to that.
Hi JR. As you posted your message 2 hrs ago, I'm guessing whatever is going to happen has already happened by now. Whatever the outcome, I hope you and your girlfriend are ok. Post back if you have questions or need support. This is the best place to find people that know exactly what you are going through. Tony