If he didn't before, he certainly knows now, after your friend contacted him! I think the guy doesn't want to have to accept any responsibility for have a "bad" line. Owning up to it would mean spending money for the vet to figure out which breeder animal might be the carrier, then taking the step to no longer breed that one and any other possible carriers, then the expense of replacing the breeder(s) animal. That would be the ethical thing to do anyway. I think it's rotten that they guy seems to have no concern whatsoever. The guy doesn't have to be like that, he could try to help track it. And, there is a slight chance that the cancer could have environmental causes. All humans carry cancer cells, but,not all get cancer. And, there are many things that can trigger the growth of cancer cells: environmental, chemical, gentics (which pre-dispose one to the virus), or a combination. I think with animals, it is very much the same way. The genetics of your friends girl most likely pre-disposed her to it. The sad thing is, that there are too many breeders out there, who realize that there are genetic defects in their lines, or illness (either in their breeders or in their clutches) and they continue to breed and pass these things on to future generations, which further weakens an already weak genetic pool here in the US. To those un-ethical breeders I would say "Shame on you!" And, if any of them are members of the forum and offended-too bad.
The breeder that your friend got his girl from should be ashamed of himself for not being willing to at least give your friend whatever info he has and for not being willing to take steps on his own to prevent further generations being born with such viruses. (sorry, that turned out to be a little bit of a rant!)