Here are two functions that will pick up from email and subject from what the user entered – you can change them to suit your needs. Put them in your functions.php.

// Avia Contact Form related functions
add_filter('avf_form_from', 'avia_change_from', 10, 3);
function avia_change_from($from, $new_post, $params){
        if (!empty($new_post['e-mail'])) {
                $from = urldecode($new_post['e-mail']);
        }
        return $from;
}

add_filter('avf_form_subject', 'avia_change_subject', 10, 3);
function avia_change_subject($subject, $new_post, $params){
        if (!empty($new_post['subject'])) {
                $subject = urldecode($new_post['subject']);
        }
        return $subject;
}