Without more information about your friend's medical history---and that of her family---it is impossible to determine what really happened to her. While it is possible that a clot formed and caused a heart attack
, the full explanation is much more complicated that that. She may have had any number of various medical disorders that contributed to what happened, and all of these disorders are extremely rare. These include congenital heart disease
, congenital rhythm disorders, inherited clotting disorders, and a multitude of other very rare disorders. Because of the relative rarity of these disorders, it is highly unlikely that a similar thing would happen to you or anyone else that you know.
I am sorry for your loss.