I have, and it sucked.
Friend K I had known longer, and friend S was the girlfriend of mutual friend between myself and K whom we'll call St.
K and S started clashing really bad, because St wanted me and K to be friends with S as his girlfriend so we could all go hang out together and stuff coz he was nice and sociable like that.
My friendship with K was a bit rocky at that point anyway due to unrelated circumstances and peoples, and it quickly came down to a point where K stood her ground and told both St and myself and it was her or S. As much as St valued K's friendship he chose his girlfriend, and as S had been a better friend to me in those times I reluctantly told K that I'd pick S as well.
In the end St and S broke up and I'm no longer friends with S but I'm okay with that, honestly. My philosophy is that if it comes down to making a choice between friends then something has brought it up to that point and the universe is trying to tell you something and it's better to just make the choice and go with the flow than trying to make things work out.