Contemplating moving on from marriage, last night was pretty much the final straw M50, wife F44, friend late 50's early 60's (we don't know for sure)?

So I (M50) my wife (F44) have been together for over 20 years and our friend (M late 50's early 60's? never brought up his exact age just guessing) that my wife met through work and introduced me to have known him for at least 9 years went out last night, they had some awful tasting mixed margaritas and I had a couple of lighter drinks. We got home and I went to bed, but I was woke up by some sounds coming from the living room, found my wife giving our friend oral.

Now the weird part is that they acted like I caught them in the act and I went back to bed (I didn't sleep at all, but I sat in bed, I was looking up divorce lawyers was my first reaction), but later this morning they were acting completely like nothing happened, our friend went home, I then brought up with my wife what I heard and saw and she turned white and swears she doesn't know what I'm talking about?

I mean I know if they did remember being caught our friend would have been straight out the door and ashamed, but even he is texting like he is completely unaware of what happened after we got back home to both me and my wife, their drinks did taste like trash, were the drinks spiked the only person that could have done that was the bar man?

Our marriage has been on a bit of a spiral of late (I'm partly at fault in this I'm aware of that, but I never went and sought physical contact with anyone), so this latest thing has got me contemplating looking for a separation and moving somewhere completely different. I really don't know what I should be doing right now?