Because I have received messages and posters have expressed surprise, I will clearly state and, if possible, clarify what I said on this thread.
I. Any active participation in a non-Catholic rite, whether heretical or schismatic, is a grave offense to almighty God, and a scandal. The Holy Office, Canon Law, the Fathers, et al. are quite clear on this, and it is a violation of divine law.
II. The Pio-Benedictine Code of 1917, which I regard as binding, allows, as Fr. Carl Pulvermacher says, passive attendance at non-Catholic services "due to one's position or for politeness, provided the danger of perversion or scandal is precluded" (The Angelus, July 1983). The canon states:
Sec. 1 It is not licit for the faithful by any manner to assist actively or to have a part in the sacred [rites] of non-Catholics.
Sec. 2 Passive or merely material presence for the sake of honor or civil service, for grave reason approved by the Bishop in case of doubt, at the funerals, weddings, and other solemnities of non-Catholics, provided danger of perversion and scandal is absent.
Now, I gave Ancilla Domini the benefit of the doubt in this thread. As far as I know, since I do not have access to the interior forum, she is a Catholic in good standing, professing the Faith, free from canonical impediments. She states that she received an invitation to attend this said liturgy, which falls in line with what the late Fr. Pulvermacher said, "politeness," or the "sake of honor" stated in Canon Law. I do not think attendance at this rite, even if given undo enthusiasm or gusto, enables me to jump into inquisitorial mode - why are you going, who invited you, what's going on, don't tell me you're going orthodox, you know you can't receive communion, etc. - given that she is (a) a Catholic and (b) has not given any signs where such fraternal correction is obviously necessary. If she attends because there's no better option, I would be troubled and not recommend doing so.
I seldom answer to anyone save those who have actual authority over me, whether in the temporal, familial, or spiritual domain, but I choose to do so now, lest I am accused of misleading others in my position as a moderator of this Internet forum.