Imagine that a person is about to die of natural causes and is aware of it. They are not experiencing any physical pain and the thing that is giving them peace of mind is the belief that their immaterial soul will be reunited with its supernatural creator; probably also that they’ll go to heaven because they’ve “been good”.
You make them aware that the soul is material and that no thing that is immaterial exists, that good depends on your take of what is pleasurable or not, but you realize that rather than bringing them more pleasure to these final moments you produce them more mental pain.
A false opinion that brings pleasure rather than turmoil.
Being truthful for the sake of it at a moment like this seems stupid and not likely to produce any pleasure to either party.
Is the value of truth relative?