Testa att lägga till en groupBy i din fråga (otestad)
$select = $this->_db->select()->from('reviews')
->joinLeft('users', 'reviews.reviewer_id = users.id')
->joinLeft('websites', 'reviews.user_id = reviews.user_id')
->where("reviews.user_id = $user_id")
->group('reviews.id');