If you unpack the question, in a sense you’ve already said, “Here is a new believer, somebody who has trusted in Christ.” If they go through some traumatic accident that makes us feel they are beyond our reach, then there’s no reason that we should doubt their faith any more than Jesus doubted the faith of the dying thief on the cross. He did not have long to produce the good works that are the fruit of faith.
The second thing to say is that there are mysteries to human existence. When a catastrophic accident happens to somebody, we should be able to trust that if they have trusted in Jesus Christ, then they are resting in Him.
I sometimes think of the 23rd Psalm in this connection because something interesting happens. The psalmist says, “The Lord is my shepherd; I shall not want” (Ps. 23:1), but then the dialogue changes when the psalmist goes through the dark valley. The person reading the psalm is not hearing the psalmist give testimony, but rather hearing the psalmist in this hidden place where he’s engaged in fellowship with God. We are not able to see into that dark valley, but there is an intimation that comes out of that dark valley that there is still fellowship with God.
It’s really worth reading through Psalm 23 for this reason. I don’t think it is always noticed that you are now overhearing something that is beyond your sight and beyond your reach. I’ve sometimes thought of that in connection with believers who have begun to suffer from dementia, or Alzheimer’s, or those who have no capacity to communicate to us what is happening concerning the engagement of their soul with God. I think that is really worth reflecting on.