This is a tough one. I have been in this situation and my first instinct was "tell them" but when I told the person she ended up calling me a lier and I lost a good friend over it. Years later she ended up in a divorce because she finally caught him herself. I personally would not do this to someone who told me my boyfriend was cheating. I'd investigate it and find out if he was infact cheating and catch him red-handed. So maybe just start a conversation with your friend on what they would do if they were confronted by someone who thought they where cheating before coming out and just saying it happened. Good luck.
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