I'm so sorry OP, this is truly gut wrenching.
I was put on the 2 week pathway last year for what my GP suspected may be lymphoma, the week before my scan my H and father of our children left in the middle of the night to be with an OW that I was oblivious about. I woke up to a text saying he had been cheating on me and has left. To add insult to injury he locked me and baby in the bedroom before he left in what I can only assume was him making sure he could get out of the house without being confronted if I happened to wake up.
Our youngest was just three months old at the time and we had a disabled toddler sleeping in the next room, left to his own devices with me unable to get out of the bedroom for the best part of an hour the next morning. I had to damage the door.
Like in your post, the bastard had been telling OW all about my investigations and went as far as outright telling her I had cancer despite no diagnosis being made.
Fortunately it was ruled out and I was diagnosed with something much less scary, but I will never forget the sheer terror of facing the possibility that I may have cancer, then discovering what my bastard H had done, all within such a short space of time. I became physically ill with stress.
The only reason I'm telling you all of this is because I can testify to the fact that him sodding off was the best thing that had happened to me for a long time, once I got over the shock.
I can understand why PP thinks it would be a good idea to keep him around for the practical aspect but I genuinely think you will do better with him gone. You need every ounce of strength both emotional and physical to fight this and he is going to be nothing but a distraction and hindrance.
I echo what others have said in making sure you tell everybody exactly what he has been doing in your hour of need. Let him feel the shame he deserves.
The OW is a piece of shit too for entertaining him in the first place but especially now she knows what you are going through and isn't kicking him to the curb.
I'm so very sorry for everything you are going through and I'm sending strength for your upcoming treatment.
Lean on those you trust and who love you
You've got this 