Can you let me know how many grams of saturated fat should a 15 year old girl with high cholesterol have a day?

Depends: Fats should make up 25-35% of the total # of calories so it depends on her size and weight. Let's suppose she's 5"4" and weighs 120. She burns about 2000 cal a day and can eat around 500-600 cal of fat. There are 8 cal per gram so the math is: 62-75 grams of fat per day of which no more than 20 grams should be saturated fat. These are approximations based on speculation regarding her size/weight.