To Tell or Not To Tell a Cheating Boyfriend

That is the question. Whether I should tell my friend I caught his boyfriend cheating on her or not to tell. My friend is suppose to marry the bf next year, and one day, I saw the man with another girl who looks like my friend but on the smaller side, medyo chubby kasi ang friend ko eh, pero this girl is kamukha talaga, payat pa. Parang smaller version, kamuntik ko na silang lapitan kasi akala ko nga siya yung friend ko.
I am with my husband, he recognize the bf too, pero siempre siguro gusto pa niyang ipagtanggol, baka naman hindi raw siya. Nyee. So we decided not to tell, baka naman hindi nga siya, is there harm in not telling? Sabi nga nila, what you do not know won't hurt you. Haay, sana lang it won't happen to me, I do not want to know.