How to update the view of an app on ios device b from device a

How to update the view of an app on ios device b from device a

I have built a webservice that handles all data inputted and outputted for
two seperate apps yet closely related: 'Sales' app, and an 'Admin' app for
a particular company.
Every time a sales assistant takes an order over the phone and inputs it
into the sales app closing the deal the admin app should be notified that
a sale has been made. (sales are not made frequently since they are bulk
sales). The first thing that popped up in my head was to use Apple's push
notification service since this would alert the admin app if the admin was
closed. Im sure that is correct.
Here is where I start to feel uncertain. What happens in the situation
where the admin app is already open, how can the admin app be updated as
soon as a sale is made? I understand that push notifications can still be
received when the admin app is open and therefor be notified right away
when a sale is made on the 'sales' app.
Talking only in terms of when the application is open, is there a way for
when the admin application to be notified straight away as soon as a sale
has been made; without using apple's push notification service; and
without regularly refreshing the admin's app page where a webservice
function would be called at regular intervals to see if any recent sales
have been made, which i feel is not a good way forward?
Or is the correct and appropriate way to use only the push notification
service?