Should friends speak up about an affair?

If your other is cheating on you, and all their friends know...do you think it is their responsibility to tell you or to warn you?

Looking back, I remember this time that SI's friend Keith came to our new house.  We were sitting outside drinking a beer.  We, or rather Keith & SI, were talking about the bartender at the bar they go to and how she lets them go behind the bar and make drinks and serve people.  This is not the first time I heard about this person - I heard a little about her over the course of the last year or two.  When we were talking outside on our porch, I remember this moment where the two guys realized they said something they shouldn't have...and I thought that Keith liked this girl.  Turns out she was SI's mistress.  I believe the three of them even went to Boston one weekend before I moved into this house with SI.  So I wonder:  If SI's few good friends knew he was carying on an affair, should they have warned me that it might not be the smartest thing for me to give up the life I knew...my FREE crappy basement apartment, my town, etc to move to an area that was farther away from my family to where I knew no one, to live with the man I loved? Only to sit in our house, alone most of the time while he was gallivanting with his other lady?  They knew about this for a while, yet no one said anything to me.  Sure, it might not be the easiest thing to say outright; but what about dropping a hint?  Could Keith or his other friends said something like 'Denise you should really double think this' or 'Denise, are you sure you know where he goes when he isn't with you?'  When I didn't hear from SI and became concerned, I'd message SIs best friend asking if he heard from SI because I knew that there was an accident on a highway.  I was always worried about him; and yet this other friend said absolutely nothing to me that SI was safe - in bed with some other woman. 

I wonder if one of these two guys did tell me...what would have happened? I wouldn't have moved to NJ, I would have saved money living in that free apartment a bit longer...which means I would have paid down my graduate school loans, I would have less credit card debt & I'd be able to afford cable TV.   I like my apartment now (not LOVE, it still isn't ideal & I am more broke than ever), and I wouldn't be here if all those things didn't happen; fate is funny. 

Sure, the confession of his friends would have caught me so off guard and would have made me so angry...but I had those feelings when I found out on my own anyway. If they could have prevented all that extra hurt, the 12 months I was so sad, confused and alone I would definitely have rathered someone to tell me. I would have wanted to hear it to prevent all that useless time spent with someone - so either he would have made the choice to call the affair off or to break it off with me.  And what did they care? We weren't friends, they weren't risking a friendship so it would have been a lot more emotionless on their part.  Also, did his friends tell him he shouldn't be doing that? Did they think about what he was doing to me at all? Did they warn him that he wouldn't get away with it forever? Or were they 'guys' in the fact they admired that he was able to carry on this double life for 2+ years?

I suggest, if you are ever in this situation, that you do speak up.  What is the worst thing that will happen?  The person you tell will get very upset with you.  You may be risking a friendship - but if that friend can't understand why you spoke up, then they aren't the kind of friend you'd want anyway.  They may not speak to you for a while, but it has nothing to do with YOU...they will be more hurt by the situation and I am sure when things settle down, will reach out to you and thank you.