How to support someone whose girlfriend has cancer?

I'm in a very complicated situation. My ex-fiance recently left me for a friend of ours that he had been cheating on me with. Obviously I have very strong negative feelings for this woman, and as he is the father of my child, I am still in love with him and we are trying to maintain a friendship. The woman that he is with now went through ovarian cancer twice when we were in high school and this week found out that her cancer has resurged. She is going through chemotherapy and radiation for a third time and had already had one ovary removed. She has been very sick and they are not sure whether she will be able to fight it this time. I have such mixed feelings about this. I am bitter and angry that this woman slept with my fiance, but even in my most hateful state of mind, I would never wish cancer on anyone and I am praying that she will recover. My question is, what can I do to support my ex through this? I have been very clear about the fact that I still love him and want our relationship back, and my angry feelings toward her. I don't know how to help him as a friend and make it clear that I am genuinely sorry that he is in pain and that I hope that she gets better. What is the best way for me to be there for him?