As long as it was me that they were getting intimate with, I wouldn't say a thing lol jk I would have to tell.
Really, the bad situation. Well, I wouldn't tell this your friend. However, if your friend asks you about it one day, i would tell him/she as it was. I've never got same situation, but that's my view on this.
I think that it depends on the situation. If your friend is open or broad minded and he/she is okay with all this, then it won't be a problem. But if he/she is narrow minded and does not likes all this, then it could surely be a situation to think on what to answer. So far I have never tried all this, so the question of getting caught does not arises. But if it happens someday (by mistake), I will post the reply here
i Would definitely tell About it If I am Sure He/she Is Not her/his brother or sister, and Without Knowing If They Are more Then friends