Remove previous code and add following code in your child theme’s functions.php

add_action( 'init', function() {
    if ( class_exists( 'Fep_Email_Beautify' ) ) {
        remove_action( 'wp_loaded', array( Fep_Email_Beautify::init(), 'announcement_email_send' ), 12 );
        add_filter( 'fep_filter_before_announcement_email_send', array( Fep_Email_Beautify::init(), 'filter_before_email_send' ), 10, 2 );
    }
}, 12 );

This is a non standard solution. This may break in future.