woocommerce_set_free_shipping_for_certain_users

function get_user_role() {
    global $current_user;

    $user_roles = $current_user->roles;
    $user_role = array_shift($user_roles);

    return $user_role;
}

function woocommerce_set_free_shipping_for_certain_users( $methods ) {
    global $woocommerce;
    $user_role = $this->get_user_role();

    $shippingrate = new WC_Shipping_Rate();
    $shippingrate->id = 'free_shipping';
    $shippingrate->label = 'Free Shipping';
    $shippingrate->cost = '0';
    $shippingrate->method_id = 'free_shipping';

    // Free shipping rule If user is a wholesale buyer and order is over 150 OR if user is a retail buyer and order is over 25
    if ( ( $user_role == 'wholesale_buyer' && $woocommerce->cart->subtotal_ex_tax >= 150 ) || ( $user_role != 'wholesale_buyer' && $woocommerce->cart->subtotal_ex_tax >= 25) ) {
      unset($methods);
      $methods = array();
      $methods['free_shipping'] = $shippingrate;
    }
    return $methods;
}