One of my friends wanted me to leave out my other good friend and not tell her about our beach trip we were planning. I told her that I wouldn't go unless we could all go together because I just don't leave people out.
That's terrible. I'm glad you stood up for your friend.
I am fiercely loyal to my friends and don't think anyone should be excluded, especially among a group of friends that are all going but one person.  Not cool.  
Why would your friend want to exclude your other friend? Definitely not cool.