read this please i need advice!! only if ur 13 and up!

i have these 3 friends Ryan,Kristen,and amber. Kristen amber and myself like Ryan. then Kristen started to go out with him.so now there dating. last week my school had 2 half days and on the half days Ryan and amber hung out. i stopped liking Ryan for Kristen's sake. but last Sunday Ryan Kristen and i were at Kristen's house hanging out like nothing..then this morning i called amber because i couldn't fall asleep she told me that on the 1/2 day Ryan fingered her and she gave him a blow job and had job and asked me to promise not to tell Kristen so i promised..but its tearing apart knowing Ryan cheated on her with one of her "close" friends. I'm afraid if she finds out that this can ruin all our friendships. i don't know if i should tell but it seems like the right thing to do but i don't know anymore?