Proverbs 22:6 — "Train up a child in the way he should go: and when he is old, he will not depart from it" — is one of the most debated verses in the Bible for parents of struggling adult children. The Hebrew derek — 'way' — can be read as the child's own way, the way suited to their particular nature. The promise may be less about a guarantee of outcomes and more about the endurance of what was planted. What was faithfully planted is not gone because it is not currently visible. Proverbs speaks of long arcs.
Romans 8:26–27 addresses the condition of a parent who does not know how to pray for an adult child in crisis: "the Spirit itself maketh intercession for us with groanings which cannot be uttered: And he that searcheth the hearts knoweth what is the mind of the Spirit." The Spirit takes inarticulate grief — the groaning that does not become words — and translates it into intercession before God. The parent who cannot form a prayer for their struggling child is not outside prayer. The Spirit is praying in the groaning.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.