This is the stupidest idea I've heard in a long time.
This is just so wrong on every single level. I'm at a loss for words as to just how morally and ethically bankrupt everyone involved, including the OP is.
I hate cheating more than most people, having been on the receiving end, but this is their problem, not yours and a stranger/anonymous tipster trying to destroy their life is not going to do any good. Best case scenario, you cause catastrophic harm to two people, more if children are involved and family gets dragged into this.
There are so many ways to rightly handle this and solve their problems but you've chosen the absolute stupidest way I've ever heard of. Your way of handling it is what I would expect from some 16 year old with no idea how to handle delicate situations.