IMO, A dojo is not responsible for rehabilitating people nor is a place to "experiment" with people. As Mr. Little said, most are not run by Psychiatric or Medical Doctors that specialize in this field. Even if you have such an individual, it is not a clinic, nor is it a place for treatment.
As one of those trained in both arts (martial and psychological). training is not treatment. If the individual was asked to leave but is back, and you choose not to train with them, the Sensei will probably already know why.
And, the best way to understand why people do what they do, its a statement of who they are, not who you are. Don't take it personally.
Now get back on the mat and don't let other's problems interrupt or interfere with you training.