Having experienced in the past, the confused couples. (Are we swingers, or are we poly, or are we both ? phenomenon.) It isn`t much fun to deal with those growing pains. It can be difficult to know, if they really are growing pains, or some easy lines used to get what they want.
We decided just to chalk it up to experience, and not be taken down that path again.
If I were to say anything, I am probably more liable to say something to the 'ex' about making sure they treat the new couple properly, then go out of my way to say something to that new couple. Going out of the way to say something to the new couple, will always be countered by others, as 'sour grapes'.
Just the mention of sour grapes, will cancel out the advice offered, in most people`s eyes.
Many times unfortunate, but true.