if is_bbPress register jquery

Completely out of any context (since it’s really not specified in the question), the correct method to enqueue jQuery only in the context of bbPress is to add an appropriate conditional inside of a callback, hooked into wp_enqueue_scripts. For example, the following would be defined in functions.php (i.e. not in a template or template-part file):

function wpse129696_enqueue_scripts() {
    // Only enqueue jQuery in the context of bbPress
    if ( is_bbpress() ) {
        wp_enqueue_script( 'jquery' );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse129696_enqueue_scripts' );