It's totally normal to worry, my horse is practically my child!
I will be leaving her for over a week in February while I'm on vacation. I've already thought about how worrisome & hard it'll be, but then I remember I am getting away from it all (even though I wish I could take my horse with me! lol) & she will be in good hands. :) THAT is what matters, who will be taking care of her while I'm away.
So, it's good you're leaving her with a close friend. This way I'm sure she would contact you immediately if something were to go wrong. The 5 days will go by fast, hopefully she will send you photos and videos of her while you are away. :) That will help. Plus, her being in good hands will give you a peace of mind.
My situation right now is a little different. My horse is away at my trainer's & I cannot see her every night (like I would if she was at my private barn). For almost a month I could ONLY see her on weekends. My boyfriend helped feed my barnmate's horses so now I can go see her a few times during the week too.
BUT, gosh, it was hard! Not the same as being totally away on vaca, but still, going from seeing your horse every single day...to not, is not easy!
Try to enjoy your vacation though
Your horse will be so happy to see you when you get back...they do say absence can make the heart grow fonder, right?!