"Brainwashed?" Velma couldn't help but remain somewhat detached from things. She'd asked a lot of people a lot of things in her time as a mystery solver, sometimes people were just less comfortable with all the details. It was important to keep strong mind and on task though. "Because his behaviour wasn't his own?"
Perhaps it was like a hypnotic sort of suggestion to the point; the sleeping subconscious was easily altered. Perhaps that's how these people suffered these fates, being easily susceptible to hypnotic suggestion would explain a few things if that was the case. Especially unnatural behaviour.
"Would you say your friend was prone to personality swings such as this or would this be out of the ordinary for him?"