No, your friend will blame you for telling her. I know you are trying to be a friend, but she won't listen and you will ruin your friendship. She will need you when it does come out.
You should tell your friend that he is cheating its best that she knows.
No.
you get divorced and lose a best friend or talk through it
cheating ANSWER: what about Adultery and Adulteress
Do nothing. It is between them.
by asking your best friend to spy on him
Cheating your spouse is not a good thing. It could land you into loneliness and other miserable feelings.
Your spouse teasing you or not talking is common. They are feeling left out as you are cheating on someone else.
It is very common when a marriage has some problems that one spouse is worried the marriage will fail and they become extremely stressed about it and may suspect their spouse is cheating. When one sleeps the brain is still active and all people dream every half hour to release the stress of the day, but most of people do not remember their dreams as they are in full REM sleep. It is the first few moments when a person wakes up (it seems longer than a few moments) that they will remember a dream or a nightmare. The best thing you can do is choose a quiet evening with your spouse and calmly (without accusing them) that you had a dream that they were cheating. Your spouse will not be upset because they know it's just a dream to them, but watch for body language and facial expressions. If you feel there is a chance your spouse is cheating then ask them right out. If they deny it and you are still sure they are cheating your only two options is to have a friend with a car and the two of you follow your spouse or, hire a detective for the days your spouse is not around.
You obviously listen to your best friend. your best friend wouldn't lie to you. unless your best friend has a crush on your boyfriend. if he is cheating on you, break up with him because once they cheat, they never change and will cheat on you again. there are plenty of other boys out there, no matter how much he loves you!
Stop cheating with her and forget the whole thing ever happened. You might want to bring it up in 20 years, or so, while you and your best friend are having beers and the girl is a distant memory.