I did not say it was not wrong, although we have to keep in mind that the divorced person is not Catholic and is not bound by canon law. I said it was not a mortal sin…because it isn’t, per se. I wouldn’t like it if my husband kissed somebody else, but that is not the same as saying he’d go straight to Hell if he did. Remember, too, that in this case he’s already obtained a divorce. If we got a divorce because he honestly thought we did not have a valid marriage, I don’t think he’d go to Hell for dating before our marriage was declared null, either. A very bad idea and a* mortal *sin are not the same thing.
Do not forget that the previously-married person was not previously married in the Catholic Church. I have no idea if this couple did anything but say of the previously married man: “We got a civil divorce because we felt we had no marriage. I got no decree of nullity from the Catholic Church, because I’m not Catholic, but there is nothing about my marriage that fits the Catholic definition of a valid marriage.” What, you think non-Catholics are going to jam our tribunals with investigations into their prior marriages so they can date Catholics? That is not going to happen.
It is very much valid to warn those who date someone married and divorced outside the Church that a decree of nullity is not automatic, even if the previously-married person is acting as if the attempt had been null on the good faith of the advice of the ministers in their own denomination. The Roman Catholic Church may not come to the same conclusion as their own clergy did. It happens. It is not a mortal sin, though, for a non-Catholic Christian to believe the advice of their own clergy who tell them that they are not validly married.