do you tell your friend if her man is cheating??

my partner and i have some very good friends who are also a couple and we spend alot of our free time together. however i have recently found out that my partners friend is cheating on my friend and i am torn of what to do about it. i want to tell my friend but i know that by doing so would spoil the friendship we all have together, would split them up and would also cause tension between me and my partner - what should i do?