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You can use the scope argument for that.
Try this in your theme > functions.php or in bp-custom.php

function bp_activities_user_friends_scope( $retval ) {

    if ( bp_is_activity_directory() ) {
        $retval['scope'] = 'friends';
    }
 
    return $retval;
}
add_filter( 'bp_after_has_activities_parse_args', 'bp_activities_user_friends_scope' );