E-mails are inherently insecure and should not be used for sending sensitive information (unless securely encrypted).
Not sure what you mean by "legally use its contents". In the first place, to use it in a legal sense they would need to provide provenance. i.e. They'd have to prove that it actually originated with your friend. In so doing, they'd also have to prove how they obtained it. Both would be difficult, and if they managed to prove that, they'd probably end up proving that they obtained it illegally, thus shooting themselves in the foot!