"그의 친구가 심장마비로 사망했어요."
Translation:His friend died from a heart attack.
To die 죽다 To decease(used in officiallly)사망하다 To pass away(used respectfully ie towards a loved one) 돌아가시다
by the way 돌아가시다 means literally 'to go back' or 'return' (돌아가다) with a honorific '시' suffix attached
Hm, but if I'm not mistaken (native speakers, please correct me if I am), 사망하다 is quite neutral concerning respect to the deceased person, whereas English "to pass away" is quite respectful. I would expect 사망하다 in formal contexts (it's common in newspaper articles for example). But when speaking of, say, my grandparents or the loved ones of the person I'm talking to, I would choose a more euphemistic expression (probably 돌아가시다, lit. "to go back").
Could someone get whoever wrote this an appointment with a psychiatrist? Death, death, death...