Postgres - aggregate two columns into one item

You could avoid the ugliness of the multidimentional array and use some json which supports mixed datatypes:

SELECT user_id, json_agg(json_build_array(friend_id, confirmed)) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id

Or use some key : value pairs since json allows that, so your output will be more semantic if you like:

SELECT user_id, json_agg(json_build_object(
        'friend_id', friend_id, 
        'confirmed', confirmed
    )) AS friends 
    FROM friends_map 
    WHERE user_id = 1
    GROUP BY user_id;

SELECT user_id, array_agg((friend_id, confirmed)) as friends
FROM friend_map
WHERE user_id = 1
GROUP BY user_id

user_id |           array_agg            
--------+--------------------------------
      1 | {"(2,true)","(3,false)","(4,false)"}