Should I tell Owner (in my opinion) her vet is lying to her ?

Although not exactly the same situation, I had a neighbor with three elderly chihuahuas who developed serious medical issues, resulting in all three needing to be put down within about a year and a half. Because one had Cushing's, which can cause muscle weakness, the dog could not walk or stand. He had to be carried outside and held up to pee. Of course, there were other issues, but the owner was used to providing care because she is a nurse by profession. She spoke to me and others in her family about the dogs' various health problems. But it all boiled down to her accepting what had to be done. I never told her what I would do. Instead I listened and let her arrive at the answer herself. It is a difficult position to be in, but I would never interfere or accuse a vet. Perhaps suggesting a second opinion wouldn't hurt, but I would not take action against the vet because you don't know all that has transpired and how much may be due to the owner not wanting to let go.

I would look for an appropriate place in conversation to suggest she get a second opinion.

Remember, you're only hearing half the story. The vet may have had this discussion with the owner, and the owner can't bear to euthanize her dog. This could be the less of two evils (if the owner is putting her needs above the dog's). I don't know what vets should do in that unfortunate circumstance. I don't think they want a reputation for interfering by reporting a customer for cruelty.

You could talk to the state board of vet licensing to see if you could file a complaint. But, it would likely come back on you. The vet might actually want to have this discussion with the dog's owner. A complaint would be the impetus for it. ("You're dog sitter filed a complaint against me. And, you know.... she has a point...." He would be the "good cop," and you would be the bad.). If you spoke to the licensing board, they would probably tell you dogs are governed under property law and as long as the "property" owner wishes to keep their property....

Maybe the best thing to do would be to google for "how do I know when it's time to euthanize my dog," find a few articles and refer her to those. Mail it to her so you can compose your thoughts and present it in a way that is compassionate toward her. (It would be easier for me to mail it than to have the verbal conversation. I would say something wrong, overlook saying something helpful, etc.).

You're right that the vet has a conflict of interest. I don't know if I would mention that. It's liable to make her "choose sides" and not listen. He's telling her what she wants to hear (perhaps).

I don't think you should tell the owner that you think the vet is lying. Like Mark said, you're only hearing half the story. I'm a pet owner who had to put down my dog that I had had for 12 years, and that decision is a very difficult one. Something my vet said stuck with me and might help in your situation. He said to consider the dog's dignity and quality of life. He doesn't want to live like that, doesn't want you to see him like that, and doesn't want to treat you like that. He doesn't have much of a life if he can't walk without pain and is barking all the time. You might want to start the conversation with something like you are concerned because the dog is barking all the time and apparently has trouble walking. Then lead into what my vet said about quality of life. Then let it go. The owner has to make the final decision. I feel so much empathy for you! Good luck!

No, never, unless you are a better veterinarian with credentials. People choose their own vet for reasons that are personal to them. Undermining that relationship is hurting the owner and the dog. Plus, your opinion is just that. Why insert yourself into the issue uninvited? I suppose there are imagineable exceptions, like if the dog owner "specifically and directly asks you". Share honest experience if you must but I think we should offer comfort and support, not advice and complications, you know? And what if you are wrong? Good chance that you don't have all necessary information or qualifications. I don't think sitter - parent is exactly like friend - friend. Neutral is best. You can always vent to us! Bless your hear the for caring so deeply. Lucky dogs.