Except you have no idea that what she’s saying is true. Telling an affair partner that the wife (and in this case the husband) hasn’t loved them for years, it’s a sexless relationship, that they’ve emotionally checked out and were lonely is a tale as old as time and an excused used to justify having the affair.
We don’t know anything about the husband. In the same way that when a woman comes on here and says that her husband is having an affair we don’t know anything about her part in the affair, but she is given the support because he is the one having the affair.
It’s IMO the cancer diagnosis which makes things worse here. It would be a lot easier to tell the OP that she needs to tell/leave her DH as well as the OM if this was an affair which had been going on for some time. She absolutely would have been given a hard time, but equally she would be told that she needed to face up to what she is doing.
But this is someone who doesn’t want to end the affair or her marriage. but feels she has to not because the affair is wrong, but because her husband has been diagnosed with cancer.
And posters are actively supporting her in continuing the affair to the extent she’s being told that it’s a good idea to continue because she’ll need someone if he dies.
There’s acknowledging someone is in the wrong, and then there’s encouraging them to continue to screw around and actively anticipating someone’s death by virtue of justification.
I don’t care how much sympathy people have for the OP’s alleged position, but encouraging her to continue the affair in case the husband dies is despicable.