If your SO was cheating on you, would you want to know, and would you care who told you?
Imagine you are in the exact situation you are now, and you are in a loving, committed relationship with absolutely no suspicions, and you are happy with your relationship.
If your SO had cheated on you more than once, and was likely to do it again, would you want to know, or would you prefer to just remain in the dark? Remember, you are otherwise happy with your relationship. (This is a given, for the sake of this discussion.)
As a secondary question, suppose there was hard evidence—photographs, emails, etc. Would it matter to you who brought the evidence to your attention? Would you only want to hear this from a close friend or family member? What if you heard it from your partner’s ex, or from a total stranger?