Can dieting too fast and exercising too intensely cause fatal heart attacks in young person?

Yes: Losing too much weight too fast can lead to chemical changes in the body that coupled with excessive exercise, perhaps in a person not in good physical shape to start, could lead to cardiac issues. The best approach in my opinion is to seek medical assitance to determine how much weight to lose and how fast and if you are in good enough shape to embark on a particular exercise program.