I think this is such a tricky situation.
I think there are so many variables, including how good of a friend the person is who tells you. I have two close friends I know would not tell me to hurt me. It would be out of love. Coming from others I would have a harder time believing, and it would ruin the friendship.
I think I would want to be told.
But a situation like yours happened to me. I made suggestions to my friend...'aren't you worried that they spend so much time together?' ....'don't you think it strange that he works so many late nights when no one else in his office does'....and she never picked up on my hints.
So after those hints failed, she and I went to dinner. The conversation changed to her husbands work and I told her that here was a group of people there that are not 'husband friendly'. I explained that they all drink heavily, meet out at bars until the am, have 'special dates' and share inappropriate information with each other. On these 'special dates' the coworkers would meet up at bars or baseball games and pretend to date each other. Unbeknown to their spouses. These people are in their 30's, 40's and 50's. She knew who I meant, and then tells me her 'wonderful husband' drove this lady home after one of their bar nights (she had too much to drink.....wait didnt they all???) and the lady's husband 'found them' on her driveway and I guess their was a confrontation. So my long dragged out point is, I told her I would not feel comfortable with my husband spending so much time with this people. She flat out defended her husband and those people. She still didn't getmy suggestion.
Grrrrrr..I was at least hopeful that I planted a seed.
I was not more direct after this bc:
1) I did not see it with my own eyes
2) I thought maybe they had an open marriage.....a few times she and I had plans and she had to find a sitter at the last minute bc husband had to work, but instead he is at baseball game and his coworkers posts pics on FB....how did she not know?
3) our kids are friends and I didn't want to start trouble when I didnt know 100%
4) I didn't know what else to say to break her denial
It was horrible. I decided to put some space between us bc I felt like I was in the middle of something that was not my business and really had no where to go with it.
It is such a tricky horrible situation.
FF two years later I find her at the end of my block in her car crying bc her marriage is ending. They are separated now, and she doesn't believe half of what he has done.
It just stinks!
I felt like I tried to be a good friend and give her heads up, but just didn't know what to do after that.
Geez...I fell like I wrote a novel, LOL
ETA: I think in hindsight if I would've had direct proof I would've confronted him and given him 24 hours to tell her before I did.