I'd say the chances of him getting anything back is about 0%. It is definitely an expensive learning mistake though...
Should he file a Police report on this? He has not done so yet. Is there a chance the stranger could get nabbed when he goes to cash in at an exchange?
The bitcoin is gone, he can't get it back. Treat it as an expensive experience and move on. He shouldn't tell anyone about his seed. I don't think police can do anything about it even with the chat log(if there's any), it will just waste your friend's time and more money.