If you are an active JW, then, yes. Absolutely report to the elders! Otherwise, why would you, unless it was to deliberately hurt her? Don’t get me wrong, I believe in the sanctity of marriage and that people should work very hard on their marriage. But perhaps your wife tried for several years already to keep happiness in your marriage and was simply unable to retain it. As years go by, people change, and couples sometimes change in different directions. Its horrible to experience this, but it is far from being uncommon. The man you are today and the women she has become are, in her view, no longer compatible.
When there are no other options, the best is to divorce in good terms and be thankful to each other for the times you shared where you were happy. Surely, in those 12 years, there were such moments. Be thankful for these and understand that you now have a new life ahead of you.
As for cheating, well, if she did leave you before actually going to bed with that man, then, she didn’t cheat on you. This would be cheating only if you believed that people should never divorce. But does this really make sense? Should people who seriously try to make their marriage work only to find bitterness and sorrow not have the right to hit the “restart” button?