Seeking advice re lung cancer

We have a singe family pet, a female Manx cat, who is 13 years old.
We have had her since a couple of months after birth, having adopted her from a friend.

She has always been an indoors-only cat, and has been well cared for.
She has lost some weight recently.
The vet took x-rays and blood work, examined the x-rays and test results, and had them also reviewed by a radiologist.

Before we got the test results back, the vet had suggested we try feeding baby food to help get her weight up, and we have been doing that with success.

When the results came back, the vet and radiologist both agreed that there is lung cancer (spots on the x-rays), and that the blood work indicates there is not pneumonia.

The vet says our cat could die at any time, and is, or may be, in some discomfort.
We are not sure whether the cat is in pain, although she has probably been more sedentary than usual recently.

Our vet is a long-time personal friend, whom we trust very much, and we have no reason to doubt the diagnosis.
Our family has a strong emotional attachment to this cat.

We are struggling with what to do, including whether to have her put to sleep, and if so, when.

As far as trying to save or prolong her life, the checkup and tests ran a few hundred dollars, which we can afford, but costs over $1,000 would be difficult or unrealistic for us.

Any advice would be appreciated.
Thanks.