My labrador looks the picture of health. 3 days ago she presented with a liitle cough,then was worn out after walk today, hot weather... noted rapid shallow respirations took her to the vet. The vet had x-rays done. He said either lung CA or a fungal pneumonia.
The lungs on the x-ray looked opaque... he did not offer prophylactic antibiotics or ativan to assist her breathing as he sent us home on a death watch.

This was acute, she has never been sick, why did he offer up a 'fungal' pneumonia saying she would never recover from it. It will cost a lot and stay with her forever, he said.
Also, we know the oat cell [lung] grows fast and being small mets the easiest. She has no other suspicious lumps, he did check for that.
I'm I sentencing a good dog to death? So here I am getting a 2nd opinion.

I am not sure but my dog coughed up and sneezed up blood he was wheezing and wouldn't eat the vet said it may be cancer he died 2 weeks later.

How old is your dog?
We can't exactly give you a second opinion, but if I were you I would gather my records and films (if allowed) Most vets will send them by mail, if they are digital x-rays, even better, they should be able to e-mail them, then I would go get a second opinion.
You are well within your right to have another vet check him out.
It's not generally viewed as an insult, if you are worried about that. It is what it is, another pair of hands and eyes to assess your dog.
Personally, don't think I would have been happy to have been sent home without something to make breathing a little more comfortable for her.
But don't mention things like that when getting your records, simply state that you want a second opinions so as to do what is best for her, you have to weigh your options.