Should you tell your wife that you found out that she plans on cheating?

I suppose if you're wanting to save your marriage then sure, have at it. Just remember that when people come down to that point of moment that sometimes they always don't follow through with it (e.g. someone wanting to kill someone and could be right in front of them but doesn't take the shot or kill). You have to be certain that you know she is going to do this otherwise if you do and then she doesn't, it'll probably make things worse. You could however sit down (if you want to save a marriage of course) with her and have a nice, long chat with her and make her remember the good times of your marriage.