OMG I'm at risk of losing a dear friend I call sister of 40 plus years someone pls help.?

I rang in the new year with the best sex partner I ever had in my 54 yrs. problem is he's my best female friend's nephew and it would absolutely crush her to find out the truth that he is bi-sexual but he lied to her about who he was with in my home for new years and he started a commotion over a conversation he heard my friend and I talking about on my phone without her knowing he was listening. I guess he finally confronted her and now she's hurt but she doesn't know the whole story and I don't want to be the one to tell her the entire truth of the matter that started with him and I having sex more than once, he's 32 yrs. old so he's no Spring chicken and an adult ( I don't mess around with kids God forbid). How do I handle this situation not problem. I just don't want to lose her, he on the other hand although good in bed is regrettably collateral damage. So pls no strange or disgusting answers I will report u!!!!!