Is eating a weight/muscle gaining diet bad for a 17 year olds heart? Lots of protein and carbs (cholesterol)

Glad you asked. Consider yourself a grown man now. I'm please you want to put on muscle, and of course this will work if and only if you also do some hard-core resistance training several times a week. Diet isn't key here; just get enough protein, about 1 gram a day per kg of your current body weight. You don't need a lot of extra calories; being careful here & enduring some hunger gains your definition.
Depends. There are three main kinds of basic food categories; proteins, carbohydrates and fats. Having a mix of these is best, but some people need extra. For example an athlete who exercises intensely needs extra carbs to burn before and during exercise and extra protein to rebuild injured muscle after exercise. If the 17 year old is doing heavy workouts get some advice from your doctor.