Well, I acted as though I wasn't at all suspicious. I replied to the post as though it truly were a different member and not him.

I simply asked him the question: why still mourn the loss of someone three years later, who cheated on you, while engaged, and who clearly didn't give a damn about you or your feelings.... I said that she did a very crappy thing to him, betrayed him and showed her true colors... so, move on and find someone who will treat you right, who will be faithful and honest.

IF it is him, he will have no clue that I am suspicious it's him.