This is something I'm not sure how feel.

In our apartment two weeks ago I entered to hear noises of sex coming from our bedroom. Rushing along to the room, I saw the dor was wide open and inside was my wife and someone who I've known my entire life.
They were both naked and my wife was taking his cock deep up her asshole, as he was thrusting up into her.
It was one of those surreal moment in life you just can't get your head around. My beautiful wife was moaning out loud to what I knew was growing orgasm inside of her, and fucking her so wonderfully was a man I love with all my heart.

That man is my father, and I was watching him screw my wife. I knew he found her attractive and I also knew my wife and he had flirted in the past.
Nothing to me at that moment could have looked more amazing. Her stunning body was rising and falling, and my fathers hands were massaging her perfect breasts as she finally climaxed on his cock. Watching her writhe about as the orgasm ripped through her made me feel lustful, not jealous as it should have and I knew then I wasn't going to burst in and spoil what they had together.

There is another reason for that too. My father was diagnosed with terminal cancer eight weeks ago. And I knew watching him fuck my wifes ass, it was something he desperately wanted to do before he died.
I did overhear something a few weeks back when my wife was talking to him on her phone. She made some comment about him visiting her when I was at with work. But it was her giggling and her saying "I'm sure you'll make it. It'll be nice to see you come around". Now I realize she actually said something like "I'm sure you'll make me cum".
Not long after my wife had orgasmed on his cock, I saw my father thrusting harder and faster and then heard him shout out as his cum erupted up my wife's ass.
It was then I decided to leave them to it and walked out. My emotions were in turmoil and I couldn't get my thoughts together. On the one hand my wife and father were cheating on me, but on the other, she was giving a dying man his wish.

I'm not too sure if they'd had sex before, or since. But I know they're not now as my father is in last days of life. I could confront them both, but and this is where I'm not sure about what to do, I could also forget i ever saw them and let them have their secret. After all it'll be only me that knows and I'm fairly sure she wouldn't cheat on me otherwise. Or am I just being naive.
God I wish I knew what to do.........................