That is a difficult question but I believe that you should be loyal to your friends above all else. I would tell them because they should hear it from someone who cares about them. I think it would be less painful in the long run. He or she may hate you for it but at least you know did the right thing. I had this situation a few years ago and my friend and me have never been closer because she knows I am there for her. staying out of it is just a way to shirk your duty.