Meet him for lunch
If I were you, I'd say yes to lunch. Enjoy the fact that the former BMOC is now cap in hand to the former LMISS (Little Man in Social Siberia). With a cool curiosity, I'd let him say his piece and try to assess him dispassionately.
If you decide he's as hateful as he always was, you have at your disposal a particularly nasty tool of torture. Over dessert (if top New York lawyers eat such a thing, which I doubt), you should obliquely refer to the cheating incident. Don't confront him directly, as any halfway decent lawyer will be adept at wriggling off the hook. Instead, find some way of gently dropping a hint to show you haven't forgotten about it. He most certainly won't have forgotten either; and won't at all enjoy the feeling that his lunch guest does not view him as a legal god, but persists in seeing him as a nasty, sneaky cheat.
Hopefully you still dislike him
For your sake, I hope you do still dislike him. The alternative - that you come away thinking that he's a perfectly nice guy - would be a less satisfying outcome. To nurse feelings so strong and for so long is a serious commitment; almost an act of faith. To discover it was all for nothing could simply prove too disorienting, this late in the day.