Let me ask you this and then you will know the answer ..

If your partner was cheating on you and your close friend new about it would you be upset if you found out they new and didn't tell you ?

Unless you know for a fact and show proof , so it dose not back fire on you then yes tell the person who doesn't know there partner is cheating on them .

When you know something is not right way sit back and say nothing ?
That's not a friend looking out for them