One interesting thing about grief and beliefs like this is that one method of helping a patient is to believe them - if that's their reality then you want to work with them with that as the starting point, then move them towards a healthier reality. Like if they were hallucinating cats, the doctor would believe them and ask them to verify they exist (Can you touch them? Do other people see them? Can you take pictures of them?) and coping mechanisms for when they don't.
If he believes/agrees that her son has reincarnated it doesn't mean that he literally believes Taiga reincarnated. It could be that he believes her grief is that deep and that she's using this as a coping mechanism. I'm super excited to see how he deals with this.
