How exactly does smoking crack cause someone to have a heart attack?

My friend had a major heart attack(she's 39 yrs old)-and actually "died" 3 x -she had been smoking crack the night before-she is an addict trying to stay clean thru NA ( like me -only Ive been clean 5 yrs and never did use crack) I dont know a whole lot about how crack affects the body.What would happen if she were to relapse again?