Even if someone went browsing for it, yes that's illegal but there's no benefit in their therapist reporting them for just visiting terrible websites.
But also there are definitely ways to get accidentally exposed. That's an absolutely awful thing to call the cops over.
I think you're demonstrating incredibly poor judgment here. CSAM is a crime with real victims. Even if your patient came across it innocently, someone is out there intentionally distributing it and that needs to be investigated.