Sometimes. It's certainly going to depend on how deep the mental problem is. We've had people here in retreats who we've told to leave, because we discover that their mental problem is beyond us, and/or that a silent retreat was not appropriate. We've had people whom we've refused to allow to do the retreat because we knew their mental problem was similar. This is not to say, though, that meditation might not help either of those people, but doing an intensive, meditation retreat, was definitely not the right thing to do.
An intensive retreat intensifies mental problems. For people who have some sort of deep addiction that they are psychologically very much affected by, a meditation retreat like this is not always appropriate and beneficial.
People addicted to alcohol have done our retreat. Many times they try, but they quit. Smokers, they're one of our easy ones for the most part. So addictions to chemicals, physical addiction alcohol, drugs, whatever, those sort of people can usually do our retreats.
But when it's a mental problem like schizophrenia? No, this retreat's not suitable for that, it's too intense. Meditation by itself in a more gentle way? Yes, that can be used by schizophrenic people. Doing it in a softer way, not intensive, not in silence, and that sort of thing.