As it happened, Bilbo came through the surgery just fine and resumed normal eating, although we did give him canned food from then on. It was some time after that that we found out where the lymphoma was, as a large red mass appeared in his left ear. We pretty much assumed that there wasn't anything that the vet could do for it and it didn't seem to bother him, so we left it be.
The problem with that was that periodically it would bleed dramatically. That could be dealt with. It's now to the point where it leaks fluid fairly continuously, leaving ichor wherever he goes, including on his head. It also occasionally mats in his fur, where daisy_knotwise has had to cut them out, giving Bilbo a rather patchy appearance. We had to start locking him in the downstairs bathroom at night to keep him from crawling on top of a sleeping Gretchen for warmth in the winter.
At this point, he could play Mrs. Norris in any Harry Potter film, given how disheveled that cat is supposed to look. With Ron's luck with familiars, obviously someone should gift him with Bilbo. But I digress...
We thought that the tumor would eventually take him, but it hasn't. And he can't continue to have the tumor. It won't work.
So tomorrow, Bilbo is going to the vet where they will try to do something about the tumor. And he may not wake up from the anesthesia.
Bilbo was Gretchen's mother's cat. He came to us when she did and has outlasted her by nearly seven years.
And tomorrow, the outcome is not likely to be good.