Please add following code in your theme’s (chilld theme’s if you are using) functions.php

add_filter( 'fep_eb_email_legends', function( $legends, $mgs ){
	$legends['sender_username'] = array(
		'description'  => __('Sender Username', 'front-end-pm'),
		'where'        => array( 'newmessage', 'reply' ),
		'replace_with' => ! empty( $mgs->mgs_author ) ? fep_get_userdata( $mgs->mgs_author, 'user_login', 'id' ) : '',
	);
	return $legends;
}, 10, 2);