Do you think I Should tell my friend the truth about someone cheating on her?
Have they done something like this before?
It may seem trivial but we were friends before, and she needs to know.
It is wrong to get into other peoples relationship messes. It is nice to look out for your friends but you could damage  the friendships if they know you knew beforehand.
It's only right to tell the truth and she is going to find out anyway.
I don't see this ending well for anyone. If you get involved in the drama you could be blamed and end up losing your friends if they think you are lying or knew for a long time about it.