I guess for me, it would depend on the friend. If it was one of my friends who hate porn and find it disgusting, I, as Gia advised, would tell him right away.

If it was a friend who doesn't have anything against porn, I'd just leave it alone and let her tell him herself.

-Yellow Couch involved?

If I really, really hated the chick, I would invite the two of them, along with some other people, over to the house. When they entered, I would turn on the TV(maybe go out and rent a big screen for the occasion), hit play on the DVD player, and see what transpires from there.(there would have to be a ton of hatred towards the chick to do this though.

I would like to add that I currently have no friends who are dating ex-porn chicks(at least, not that I know of). I did have a friend who moved a few towns away, and called to tell me was dating a porn star. When he gave me some info on her, I checked it out and told him, as unimpressed as I could, he was not dating a porn star, just a girl who once did 4-5 porn movies. She didn't know he knew, last time I talked to him about it. The next time I seen him, he was with a different girl, so I didn't want to ask him about it.