bp_xprofile_replace_username_to_display_name( array $email_content, object|null $user = null )

function bp_xprofile_replace_username_to_display_name( $email_content, $user = null ) {
	if ( ! $user || ! is_a( $user, 'WP_User' ) ) {
		$user = wp_get_current_user()->to_array();
	}

	if ( ! isset( $user['ID'] ) || ! isset( $user['display_name'] ) ) {
		return $email_content;
	}

	return str_replace(
		'###USERNAME###',
		bp_custom_display_name_format( $user['display_name'], $user['ID'] ),
		$email_content
	);
}