Dude while this is my first post here, and a little off subject for what I am really here for (getting more results out of my training). I will share something, take it for what it is worth.
If you care about this person, and do not believe she will find out (thats the double edged sword) You meet both these requirements..Do not...once again....Do not ever tell her!
I have been married for 14 years , married @ 18 & 17...well almost one year ago to date she had an affair...tried to comit suicide she felt so bad about it. And with dealing with it for a year now I can HONESTLY say I wish I never knew. there is nothing but pain...will you feel better by telling her...I doubt it..actually it will be worse.
Does my wife feel better by me knowing..I doubt this, she sees the pain in my eyes all the time.
If you have to tell her.....thats what the made death beds for
Best of luck..learn from mistakes