I’m sorry to hear about your co-worker’s attraction to the sedevacantist Mass. If your friend speaks to you directly about this, or the Lord gives you an opportunity to bring this matter up for discussion, then by all means you should say something (with great charity). Pray for her, and look for ways to evangelize her.
Is it a sin to attend a schismatic Mass? Sins are rightly evaluated according to their gravity (CCC 1854), and “imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.” (CCC1735) Only the Lord knows how culpable she is.
The Pontifical Commission *Ecclesia Dei * says the following :
(Note: This also applies to sedevacantist sects).
b. While the priests of the Society of St. Pius X are validly ordained, they are also suspended a divinis, that is they are forbidden by the Church from celebrating the Mass and the sacraments because of their illicit (or illegal) ordination to the diaconate and the priesthood without proper incardination (cf. canon 265). In the strict sense there are no “lay members” of the Society of St. Pius X, only those who frequent their Masses and receive the sacraments from them.
While it is true that participation in the Mass at the chapels of the Society of St. Pius X does not of itself constitute “formal adherence to the schism”, such adherence can come about over a period of time as one slowly imbibes a schismatic mentality which separates itself from the teaching of the Supreme Pontiff and the entire Catholic Church classically exemplified in A Rome and Econe Handbook which states in response to question 14 that the SSPX defends the traditional catechisms and therefore the Old Mass, and so attacks the Novus Ordo, the Second Vatican Council and the New Catechism, all of which more or less undermine our unchangeable Catholic faith.
Pope John Paul II, in Ecclesia Dei, said, “I wish especially to make an appeal both solemn and heartfelt, paternal and fraternal, to all those who until now have been linked in various ways to the movement of Archbishop Lefebvre, that they may fulfill the grave duty of remaining united to the Vicar of Christ in the unity of the Catholic Church and of ceasing their support in any way for that movement.”