This is it SGB - her husband presumably was not dragged away by this woman in his sleep. He chose to be with her. And if she is ill, he will be devastated, too.
The OP is projecting the hurt she feels in a really unhealthy and unhelpful way. It's not the other woman's fault that the OP's ex chose to be with her. It happens. He has free will. Unless she drugged him or something.
This is the point, this is what absolutely kills (having been through it) and is the reason that some people turn it all into the fault of the 'evil' other woman - they can't bear the thought that their partner rejected them in favour of someone else.
Far easier to say it was all her fault, and thus she deserves everything she gets - including serious illness. that's just an awful way to think.
My guess is that the OP didn't think this way before it all happened, but now it is the easiest path for her thoughts to take. It's wrong, it's bad for her, and it's ugly, and it has to stop or she will end up damaged by it.
Have a little humanity, OP. She's just another woman, who loves your ex, and he loves her. she doesn't deserve your sympathy in particular, but certainly doesn't deserve this either.