How do i split up with my girlfriend nicely?

I have been in relationship with this girl for about an year now. A month ago we had an arguement and we stopped speaking to each for some time, and it was when my best friend jumped in and brought us back together, and they started speaking to each other. But then i realised they both are trying to get into a relationship. I caught my friend texing her the stuff he's not supposed to. I trield talking to both of them about what is goin on. Wel, they both apologized that it was only meant to bring me and her together, and promised they wont be contacting each other agian. Later after a week i found out they are still contacting each other and my friend's attitude has turned kinda funny with me. She's acting a bit distant. I love this women, she always been nice to me no matter what happened. But, I know realize its time for me to move on and also leave my friend behind. The problem is know she is in a lot pain, awaiting an operation in hospital and i can't carry on with this feelin what she has done to me. my question is, how do i split with her nicely in manner she realizes what she has been doing?