I see. You did say that your friend stopped accessing Top 11 through Facebook. Is their Facebook profile still active or did your friend deleted/deactivated it perhaps?

If something like the latter happened then you definitely would have issues seeing him/her in your friend list because the two of you aren't in-game friends but instead are Facebook friends. And for two managers to be in-game friends, they have to be on the same server - Facebook is basically the workaround to this.