t, MySQL - rextester

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DROP TABLE IF EXISTS Table1;

CREATE TABLE Table1
    (`text` varchar(47), `from_user_id` int, `to_user_id` int, `time` int)
;
    
INSERT INTO Table1
    (`text`, `from_user_id`, `to_user_id`, `time`)
VALUES
    ('Hi there!', 13, 14, 4),
    ('Oh hi, how are you?', 14, 13, 3),
    ('Fine, thanks for asking. How are you?', 13, 14, 2),
    ('Could not be better! How are things over there?', 14, 13, 1),
    ('Hi, I just spoke to Penelope!', 13, 15, 3),
    ('Oh you did? How is she?', 15, 13, 1),
    ('Liara told me you guys texted, how are things?', 15, 14, 4),
    ('Fine, she''s good, too', 14, 15, 2)
;

SELECT *
FROM (
        SELECT LEAST(`from_user_id`, `to_user_id`) as L,
               GREATEST(`from_user_id`, `to_user_id`) as G,
               `text`,
               CONCAT (LEAST(`from_user_id`, `to_user_id`), '-', GREATEST(`from_user_id`, `to_user_id`)) as grp,
               @rn := if(@grp = CONCAT(LEAST(`from_user_id`, `to_user_id`), '-', GREATEST(`from_user_id`, `to_user_id`)),
                         @rn + 1,
                         if(@grp := CONCAT(LEAST(`from_user_id`, `to_user_id`), '-', GREATEST(`from_user_id`, `to_user_id`)), 1, 1)
                         ) as rn,
               `time`
        FROM Table1
        CROSS JOIN (SELECT @rn := 0, @grp := '') as var
        ORDER BY LEAST(`from_user_id`, `to_user_id`),
                 GREATEST(`from_user_id`, `to_user_id`),
                 `time` DESC
     ) T
WHERE rn = 1;


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