Private Message Plugin: Change user_login to display_name

I’m modifying this private message plugin and I’ve done everything I need to do with two exceptions. One is in the message system’s header in the front end, where it says, “Welcome: user_login”, and the other is in its front-end directory, where it lists all of the users by user_login and provides a link to message them. I want to change both instances of user_login to display_name, but I can’t figure out how to call it up. Just changing user_login to display_name simply makes the that text disappear on the page. Here are the two bits of code I’m trying to modify:

(1)
$header .= get_avatar($user_ID, 60).”<p>“.__(“Welcome”, “cartpaujpm”).”: “.$user_login.”
“;

(2)
function dispDirectory()
{
$users = $this->get_users();
$directory = “”;

foreach($users as $u)
{
$directory .= ‘<p>‘.$u->user_login.’ – actionURL.’newmessage&to=’.$u->ID.'”>’.__(‘Send Message’, ‘cartpaujpm’).’</p>’;
}
return $directory;
}

I can provide a link to my site but you have to be registered and logged in to see the problem. If someone thinks they can help, I’ll provide you a basic account if necessary. Many thanks for any help!