Dear (friend's name),
I'm writing to you because I know that you're feeling a little bit stressed at the moment and I wanted to try and help you out. Obviously, it's not easy taking exams, especially when you know that it's your future on the line, but there are ways to get through this! First of all, take a deep breath. Don't worry too much about the outcome; just focus on doing your best and everything will work out in the end. The most important thing is that you enjoy yourself while doing this if you're enjoying yourself then your stress levels will be significantly lower than if you were dreading taking them in the first place! Secondly, remember that this isn't the end of the world if you don't pass then so what? You'll just have to apply again next year (or in five years' time if it's a higher degree). There's no point worrying about something that hasn't happened yet and might never happen at all!