I've found a friend after trying to overdose on percocets and I was so sure he was dead, it's such a blessing today that he was not.
I'm really sorry you witnessed that, but that is probably the worst thing anyone could ever see. The worst part is already over. It may come back to you in your dreams. But it's only to remind you how strong you are.
Try your best to take care of your friend, remind him that no person is worth his life but himself.
Much love, and much good vibes to you, your friend, and your friends fiance. May you all find sanctuary, and peace.