I would say that it is not appropriate for a teen to “admonish” a parent nor snoop or pry. But, when it is presented to them, then I don’t think it’s wrong to say so when something isn’t right.
In this case, the mother dragged the son into it by wanting him to actively participate in an invalid “marriage” as a groomsman and guitarist. Since she did so, I think he had a duty to refuse and remind her why. She has, after all, devastated his family and begun an adulterous relationship in his house.
I would say that, for example, if a parent was being lax about going to Mass it would be fine for the child to say, “aren’t we going to Mass?” and if “no” is the answer, then prompting that it is a serious sin not to go. Then if it’s still “no”, well drop it.
But, if you find your mother’s diaphram hidden in the sock drawer, well, probably not your place to bring up contraception. Now, if Mom has “the talk” and brings it up-- fair game.