Help with query

By the looks of it you are doing it 1 query by for every friend? That unfortunately creates infamous N+1 issue, if you have 100 friends, you are doing 100 DB queries.

The simplest solution is to get all users you follow, then all users that follow you, then find the overlap in PHP side.

PHP Code:

$following = [];
$followers = [];
$followbacks = [];

// get all users that current user is friends with
$q = $this->db->select('friend_id')
    ->where('user_id', $this->session->userdata('user_id'))
    ->get($this->relationship);

foreach ($q->result() as $row) {
    $following[$row->friend_id] = $row->friend_id;
}

// get all users that follow you
$q = $this->db->select('user_id')
    ->where('friend_id', $this->session->userdata('user_id'))
    ->get($this->relationship);

foreach ($q->result() as $row) {
    $followers[$row->user_id] = $row->user_id;
}

$followbacks = array_intersect($following, $followers);