Hold the press!!! Man, if a stranger sitting next to me snooped to get my email and proceeded to email me, all hell would break loose. I wouldn't give a tinker's cuss what the f*ckin! message was - the message is irrelevant. I am astonished your friend did not kick up a real fuss about the creep's violation of her privacy. What he did was so wrong on several levels. A loud protest would extinguish this creep's behavior.
BTW, if someone looked over my shoulder whilst I was on my laptop, they wouldn't be able to determine my email. I'm confused about how looking over someone's shoulder on their laptop will reveal their email details. Could you please clarify exactly what happened?