Nurses,med people-clogged artery advice,please!?

ok,my father is having a byp tommorow.he has calcium in his arteries.i never want to get this.my dad was pretty health conscious,too,so its strange.im a lifelong vegetarian,and now especially,im very conscious of eating lo-or no-fat,foods.but the nurses on this last shift when i visited today,were all talking about skinny,fit patients-one a triathlete-who needed bypes,as they said some of it is genetic,nothing can prevent it if its ur heritage.i feel discouraged.do i give up and eat whatever i want,as it will happen anyway-or were the nurses just being pessimistic?i am about 20 pounds overweight,recently diagnosed with hypothyroid.im working out and taking synthroid.can i help myself by avoiding fats,or will heredity do this to me anyway?i dont want to go thru what he is going thru.