He had no idea!

Been talking with my cousin and he mentioned he has been finding IR sites in his cookies on his computer, what he doesn't know is his girlfriend is a BBC Slut who I get to tell me her stories on a regular basis. I want her to get my girlfriend to try it. While he is at work she invites a couple guys over to suck them off and get fucked. She records on her webcam and she showed me one video, both guys were well equipped if you know what I mean. I found out cause I answered a post on an adult site for people to come watch a couple fuck at a hotel. Turned out she was the one, when she saw and recognized me she smiled and continued.

Do I let it go?
I didn't know my wife was fucking black for the last few months. I got suspicious and asked her about it this last Friday. Long story short, I am now totally cucked and she is fucking all the black cock she can handle.

So, with that limited experience, my take on it would be to simply tell that if he is curious, to ask her about it. Maybe he would love it....or not. Otherwise, it is not your place to get involved.
I have to chip in that as a matter of principle, if someone's GF is cheating on them and you find out about it, that you tell them. The messy reality is, that could put you in the middle in ways that you really don't want to be, depending on the people and the dynamic - but if you think you can be honest with them and not somehow end up blamed, then you should be. And then of course there's the practical issue that you're getting to enjoy being dished some hotness as a side benefit...but that shouldn't stop you from doing the right thing, if you can.

This IR stuff turns out to often end up messy and complex, doesn't it?