Something that I've never come to understand is why people defend their best friends, even when said best friend is destroying another person's happiness.
Men and women both defend their closest friend in times of turmoil, especially when they've cheated. You know what I'm talking about. It's been in so many movies where people who cheat confide in their best friend and expect them to stay loyal.
No. Just No.
If your best friend comes to you and says that they are helping someone cheat on their partner; you need to put an end to it.
As a person who has experienced this first hand, it's almost as bad as being cheated on. You are unknowing to what is going on. Then you are hit all at once and the worst thing someone can say is, "I wanted to tell you, but they've been my friend for.."
It makes the one who has been hurt feel embarrassed, unimportant, and weak. Every time I heard this, I wanted to scream at them.
Doing the right thing isn't hard and if you care about your best friend, you won't have their back on this one. It opens doors to so many future problems that could possibly put you in the middle or worse, they could help your partner cheat on you. Not stopping it shows them that they can get away with whatever they want and you will be there to support that type of behavior.
First, confront your best friend as to why you don't support them. Let them know that it is wrong and you don't want to be a part of that drama. This can also go for any other situation. That you have a moral standard and you attend to uphold that.
Second, go to the person who is being cheated on. Let them know that you are here for them and you don't support what that person is doing and mean it. Be there for anything that they need and stand your ground on the subject; the person will need a strong rock in the issue.
There are so many scenarios that I could describe to you to explain why this type of behavior is just as bad as the act itself but I think I summed it up pretty well.
Don't defend your best friend in these types of scenarios. If they don't understand or get mad; give it time and they will. You are doing this to show that you care about them and want them to be a better person. In the end, you are shown to be someone that can be trusted to do the right thing.