A needy roommate, classmate, relative, or other associate who impedes your ability to make friends by insisting on being part of any new relationship you try to make. The friend blocker stands next to you while you are meeting new people and makes clear that the blocker is part of any new friendship.
Dillon is introducing himself to Jeremy, a new guy on the dorm floor. Dillon's roommate is a friend blocker, so he walks up beside Dillon and says, "Dillon, my man, who's our new friend?" Then the roommate stands immediately next to Dillon for the duration of the conversation.