Make TO field [‘fep_mr_to’] Readonly

Since I allow my users to only chat with a single correspondent, previously defined when a user click on a link containing href="<?php echo fep_query_url('newmessage', array('fep_to' => get_the_author_meta( 'user_nicename') ) ); ?>">

I would like to make the TO field readonly.

I tried to add the following code in my child-theme function.php, but it doesn’t work.

add_filter( 'fep_form_fields', 'fep_cus_fep_form_fields' );
function fep_cus_fep_form_fields( $fields )
{
    $fields['fep_mr_to'] = array(
      'readonly'        => true,
    );

    return $fields;
}

What else can I do?