Veuillez excuser ma syntaxe si elle est un peu erronée, mais j'essaierais probablement quelque chose comme ça.
SELECT people.id,
people.avatar,
people.firstname,
people.lastname,
LEFT(msg.message, 90) AS message,
msg.seen,
msg.date
FROM people
INNER JOIN (SELECT message, seen, date, sender_id
FROM messages GROUP By sender_id
ORDER By date DESC LIMIT 1) msg
ON people.id = msg.sender_id
WHERE reciever_id = '". $user_data['id'] ."'