I have similar problem. The friend I want to remove simply does not remove...
I added him in FB, I remember. Removed him from FB, but he is still in my friends list in-game.
There's no X in his profile in the game. I checked few times my FB friends and he is not there anymore.
Still, no matter I've removed him from FB, he is still my in-game friend.
I want to get rid of him and I need help to do it, lol