What to do when your best friend cheat?  Unless you are connected to the best friend as siamese twins, you are to stay out of the situation.  Your suspicion can not be proven unless she tells you that she is cheating so on this one, you should mind your own business.

If you mind you own business, guess what, you can remain best friends.  Why would you ruin a  relationship between you and your best friend because she is cheating.  Her cheating makes her less of a person to you and especially to the person who she is cheating on, but the business of addressing the cheating issue is not for you.  The person who is being cheated on is the main person who needs to take action.  You should not initiate any action such as telling the person who is being cheated on.

The only thing that you will accomplish is that you will lose a best friend and now who would you go the the concert with or see a movie.  Of course friends are worth more than a concert or a movie, but the implication here is thaat your relationship will be ruined.  Think about the outcome of any action on your part and make the decision to stay out of the cheating triangle.

When your best friend cheat and you tell the person who is being cheated on, you will lose a best friend for telling her business.  Your best friend will find out that you told and that would be the end of a relationship. Your bestfriend cheating is her business and though it shows that there is a flaw in her chararcter as all of us are guilty of in one way or another, it is not your business to get involved with the relationship of two other people.

How do you know that your best friend has not broken the relationship already and is in fact not cheating but is just dating someone else.  Because you are not the go between with your best friend and the person who is being cheating on, the best thing to do is to not say anything about the situation at all. If you were a really best friend she would have told you about the person that she is cheating with and maybe asked you how to break off the other relationship.  Best friends are usually best friends forever and friends do not keep secrets or try to hide any uncharacteristic actions.  Best friends keep each other on an even level of normalcy or there is no friendship at all.

What you need to do is focus on what is going on with your love life.  If you have the time to check on your best friend's relationship, then you need to get your own life together.  Dr. Phil would say that you have too much time on your hands and will be going beyond your boundaries if you intrude on your cheating best friend..  So let the situation go with your best friend and get on with your own life. 

Cheating often indicates that there are trust issues with a person.  You may want to reexamine your relationship with your cheating best friend.  Social experts say that if a person will cheat that he or she will steal.  Keep a close watch on your belongings especially your feelings of friendship in the relationship and you may want to step out of this one.  Make new friends and make a pledge not to get involved with their personal life and limit conversations to  sharing like interests.  You will be on a good path for a lasting friendship with a trusted friend after you walk away from this one if you are torn with the question whether to tell when your best friend is cheating..