How does your friend know this happened to him?
I doubt the school would admit they even accepted anyone by accident/mistake.
Unless their scores were so bad compared to the average and doesn't have anything special, like alumni connection, URM, etc. then you have something. If they were relatively in range then its nothing more than underselling themselves.
Schools have sent out wrong acceptances before. some college in NY did it recently. Granted most were undergrads and most times they just blame it was a computer glitch, but happens and the schools all say nope sorry.