In a group of friends, we asked openly who would want to be told and who wouldn't. I was surprised at the conversation that struck up. It was very entertaining. I think that it is a good conversation for close friends to have. Then you do not have to wonder.
One thing we all agreed on was not to get involved unless it was a lot more than a hunch or rumor. I mean, you don't want to be the friend that reports a big inappropriate hug to this hot woman, to find out that is his very attractive sister from out of state here on vacation on a surprise work visit.