CCC 1861 says, Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.
Rebellion against the Catholic Church, which is the very Church of Christ, is serious matter. As the Catechism also says in 1857, For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
Though their dislike of the Church (which I can only assume suggests rebellion against it as well) is certainly grave matter, they may not have met the other conditions for mortal sin (which, as I cited, is what leads us to damnation). So, there remains hope for them. They may well be able to be saved, but you should certainly pray for them and fast when you can, offering it up for their salvation. This is something we should do for deceased Catholics, so imagine how much more we should do it for those who died apart from the Church.