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I would hope there's no service out there that lets you guess passwords like this.Technically he didn't guess the password to any specific service, he just happened to have stored his own Reddit password in plaintext as the body of a draft email. The email service allows you to search within the body even if your message is "hidden" from their interface. At worst, he MacGyvered a feature of their service to recover a string he couldn't remember.
This was a coding exercise, nothing more. If he had stored his Reddit password in some obfuscated/encrypted format behind another password-protected service, he likely would not have pulled off this stunt.