Objectively speaking, suicide is gravely sinful. It is an action of despair, one in which the person makes the ultimate rejection of God’s gift of life to us. If committed with full knowledge and consent, it’s mortally sinful, i.e. hell.
That said, suicide is often, maybe almost always the outcome of a serious mental health problem. That can mean that it is NOT committed with full knowledge and consent.
Ultimately, only God is capable of judging culpability for sins. We can know principles, but lack the capacity to apply them perfectly in situations and to people whom we can never know the way God does.
In short, they are at the Mercy of God, who will honor their deepest choices to accept Him or reject Him.