Code of my comment on Brajesh blog

<?php
/*** i commented on http://buddydev.com/buddypress/making-buddypress-activity-searchable/ ***/

/***** the src-activity.js file *****/
?>
jQuery(document).ready(function($){

    if( $('form#search-form #search-which').length ) {

        if ( '' != $.cookie('bp-activity-search-terms') ) {

            $('form#search-form #search-which option[value="activity"]').prop( 'selected', true );
            $('form#search-form #search-terms').val( $.cookie('bp-activity-search-terms') );
        }


        $('#search-submit').click(function(){

            if( $('form#search-form #search-which').val() == "activity" ) {

                search_terms = $('form#search-form #search-terms').val();

                if(search_terms.length == 0)
                    $.cookie( 'bp-activity-search-terms', '', {path: '/'} );

                else
                    $.cookie( 'bp-activity-search-terms', search_terms, {path: '/'} );

            } else {
                $.cookie( 'bp-activity-search-terms', '', {path: '/'} );
            }

        })

    }

});

<?php
/***** now lines 65 to 67 replacement in your class final code ******/

$_BP_COOKIE = &$_COOKIE;
if ( isset( $_BP_COOKIE['bp-activity-search-terms'] )  && !empty( $_BP_COOKIE['bp-activity-search-terms'] ) ) {

    if( empty($qs) )
        $qs="search_terms=".$_BP_COOKIE['bp-activity-search-terms'];

    else
        $qs .= "&search_terms=".$_BP_COOKIE['bp-activity-search-terms'];

}

?>