What is the likelihood of being punished for an adulterous act that occurred years ago?

It would be up to the current commander. As a former commander, if this isn't prejudicial to good order and discipline and morale of the current unit, just reported by ex spouse after two years (sounds like vindication versus justice), the member is a stellar performer, then I probably wouldn't pursue it. But again, I would seek the advice of the JAG before I made any decisions but it is up to the commander of whether or not to pursue.

This is a legal question, there are all sorts of things that can impact this outcome; statute of limitations etc. it sounds odd and not enough info, either way I would be blockading, you need to speak with a Lawyer. Thank you for your service.

I did something in 2005 without knowing that it occurred. Ten years later and out of the military, it came back to bite me in the butt, my legal troubles began then...the rest is history. Consult a lawyer.