You can override view-message-content.php template to manually remove that class from parent message. Currently there is no direct hook to achieve this. Next version i will try to add hook to achieve this easily.

You can make changes from this commit https://github.com/shamim2883/front-end-pm/commit/606a4b8bc1ea987721cf13c409f64cf46face71d

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

add_filter( 'fep_sanitize_html_class', function( $classes ) {
	if ( fep_get_the_id() && ! fep_get_message_field( 'mgs_parent' ) ) {
		$classes = str_replace( [ 'fep-message-content', 'fep-hide-if-js' ], '', $classes );
	}
	return $classes;
});

if that class removed then if you click toggle button, it will toggle.