Someone earlier made a good point...
Is it confirmed that he's a cheating bastard or just a really good rumor that everyone believes to be true. Confirm that before you consider doing anything.
If this is a situation that involves 'casual friends', meaning they're not so close to you that you consider them family... stay out of it.
If this is a situation that involves a friend that is like a brother to you, tread lightly... as mentioned, you don't know what is going on behind closed doors. And, despite your good intentions... it could blow up in your face. Be prepared.
But, most importantly... make up your mind and either say something or just sit on it and deal with it.
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