Dynamically change page title from URL param

You could do something like this in your header.php file (or where ever you are setting your page titles in your templates)

<?php if (isset($_GET['user'])) : ?>
<title>Entries from <?php echo(htmlspecialchars($_GET['user'], ENT_QUOTES)) ?></title>
<?php else: ?>
<title>Normal page title here</title>
<?php endif; ?>

Without more info on what plugin you are using, or how your page titles are currently being set, this is the best I can recommend