First, Cantate Domino really Is ex cathedra, if my understanding is correct. It seems to use definitional language (“firmly believes, professes, and proclaims”) and meets the other criteria: it is addressed to the whole Church, it is about a matter of faith, and it is written not as a theological opinion but as a decree.
Second, baptism by blood is like baptism by desire in this respect: neither one is an Exception to No Salvation Outside the Church. Baptism by blood doesn’t work just because you got killed in the name of Jesus, you Also have to believe in Catholicism, At Least in an implicit way. Cantate Domino is in part repeating an ancient doctrine: if you get killed by pagans but you’re an obstinate heretic, you don’t get a free pass. Just as dying for Christ won’t save you if you lack true love (1 Cor. 13:3), so also it won’t save you if you lack true faith.
Baptism by blood is not an exception to that. People who get baptism by blood Do believe in Catholicism: they are catechumens. They are in the process of converting. The Church Fathers said they are already united to the Church in a partial way: “as the catechumens have the sign of the cross on their forehead, they are already of the great house; but from servants let them become sons. For they are something who already belong to the great house. But when did the people Israel eat the manna? After they had passed the Red Sea. … [And] Jesus bring[s] over [catechumens] by baptism…[to] the living bread.” (St. Augustine, Tracts on John Tract 11 Paragraph 4)
Catechumens are therefore not outside the Church. They “belong to the great house.” When they are martyred, they are not dying outside the faith, like the people in Cantate Domino. They are dying In the faith, and That’s why they get the benefits of martyrdom.
BTW similar reasoning applies to people in invincible ignorance. The good ones actually Are part of the Church, even though they Appear not to be. St. Gregory Nazianzus supports this: “[A certain pagan] was ours even before he was of our fold. His way of living made him such. For…many of those outside [the Church] belong to us, who by their way of life anticipate the faith, and need [only] the name, having the reality.” (Oration 18:6)
There are two things at work here: there is no salvation outside the Church, and many people who Appear to be outside the Church are actually In it. This includes catechumens and good people who live in invincible ignorance. The Church Fathers said there were even some people in heretical groups who were invincibly ignorant and could thus be saved: “[Men who hold to] doctrine[s] which [are] false and perverse, if they do not maintain [them] with passionate obstinacy…but have accepted [heresies] from [their] parents…if they are with anxiety seeking the truth, and are prepared to be set right when they have found it, such men are not to be counted heretics.” (St. Augustine, Letter 43 Chapter 1)
Third, you might be interested in this link:
Scripture, Church Fathers, and Medieval Doctors on the Possibility of Salvation for Non-Catholics
Fourth, if your friend believes that baptism by blood is a heresy condemned in the 1400s, ask him if he believes Pope St. Pius X was a heretic. St. Pius X believed in baptism by blood: “The absence of Baptism can be supplied by martyrdom, which is called Baptism of Blood, or by an act of perfect love of God, or of contrition, along with the desire, at least implicit, of Baptism, and this is called Baptism of Desire.” (Catechism of Pope St. Pius X, On Baptism, Question 17)
And ask him if the Doctor of the Church St. Alphonsus Liguori was a heretic too: “baptism of desire is perfect conversion to God by contrition or love of God above all things accompanied by an explicit or implicit desire for true Baptism of water, the place of which it takes as to the remission of guilt, but not as to the impression of the [baptismal] character or as to the removal of all debt of punishment. It is called ‘of wind’ because it takes place by the impulse of the Holy Ghost who is called a wind. Now it is de fide that men are also saved by Baptism of desire, by virtue of the Canon Apostolicam, ‘de presbytero non baptizato’ and of the Council of Trent, session 6, Chapter 4 where it is said that no one can be saved ‘without the laver of regeneration or the desire for it.’ ” (Moral Theology, Bk. 6, nn. 95-7)
Also, the Council of Trent defined that a non-Catholic who desires to be a Catholic could be saved in certain circumstances. Jimmy Akin points out the relevant passages in this article.
I hope that helps. Please let me know. God bless!
BTW the reason I say to ask your friend about St. Pius X and St. Alphonsus is because it puts your friend in a dilemma. For various reasons, these two saints are typically hero-figures in the minds of the deniers of baptism of blood, and it is little known that they both supported and believed in the doctrine that these people deny. And this was LONG AFTER the supposed definition against baptism of blood took place. If they admit them as saints, then they are essentially saying one of two things: either the alleged definition against baptism of blood wasn’t a dogma, or these people rejected that dogma and became saints anyway. This would imply that you can reject Catholic doctrine and be saved anyway, which is the very position these people want to deny! There are only two ways out of this dilemma. One is to say that these two people weren’t actually saints but heretics, which would be very difficult for these people because they tend to respect these saints. The other way is to admit the truth: baptism of blood was never defined as heresy, they are simply misinterpreting the documents which they think say that.