Well, we know that spontaneous remissions on cancers do occur, very rarely, but they do occur. One of the hypothesis is that the immune system finally learns to attack the cancer. With the huge number of people who, faced with a disease that scientific medicine doesn't know how to cure, go to prayer or healers, it's not surprising that, statistically, a few spontaneous remissions do happen just after such a visit. Especially considering the placebo effect, and the non-negligeable links between the efficiency of the immune system and the mental state (it's well known that stress diminish the efficiency of the immune system).

What would be meaningful is not a single case of unexplained spontaneous healing. It's a significant, reproducible, higher-than-placebo, increase in survival rate by a given healer (or a set of healers using a given faith). And that is, as far as I know, not backed by any study (or if it is, please show me the link).