I'd ask him any questions if you know he is in therapy, it could be helpful to raise questions respectfully so he can ask his therapist for more help. If he is not in therapy, he might not be able to go to therapy or maybe something else is happening (for some reason it's become very usual to lie about having DID, or maybe it is an incorrect diagnosis or self diagnosis).

Now, DID is a very serious issue. Also very rare, since dissociation doesn't usually reach that level (we all dissociate sometimes without "fragmenting" our personality, like when we are in a car or a very long line. It is not a disorder until it becomes a problem for your daily life).

It is caused by extremely severe traumatic events, specially in childhood. It causes a person to go numb to emotions, sensations and other stuff (this is called disociation and has many manifestations) the host (the person who developed DID) is experiencing, and where alters (the "personalities") "come out" to protect the body and host from psychological damage. This is a process that takes some time to happen, not seconds, specially if the host fights to ground themselves. When someone dissociates, they can't really maintain focus on most stuff because they feel in danger, very stressed or anxious. It is also very tiring and debilitating.

Alters are created as a way to take the person away from what they are evading (not created by the person in purpose), and they can be triggered by many things, depending on the host. Alters can express themselves in many forms, and have specific purposes. They also can be "merged" with the host through a long process of therapy, or make arrangements to keep going with the most normal life they, as a "system" can.