Is it ethical to not tell a patient about a previous dentist's obvious mistake?

New patient came in. Previous dentist did a crown on #19, and it's... bad. Obvious open margin, poor fit, probably going to fail. Patient is happy with it, doesn't know there's a problem. Do I: 1. Tell them it needs to be redone (they'll be upset, might blame me) 2. Wait until it fails (feels wrong) 3. Just monitor it (but what if it causes bigger problems?) I don't want to badmouth another dentist, but I also don't want to let a problem go untreated.