Their orders will be grouped under their account and they will be able to see them every time they log in to the back end.
You can also export Orders in a CSV file filtered by Partner account.
Partners can process online credit/debit card payments via Stripe through the back end (with customer's consent of course).
To make user a Partner go to the back end > Dashboard > Users > Edit user > User is Partner: Yes
Note: Users can not be both Additional admins and Partners.
Partners will be able to access the back end Orders (only their own orders) and Users sections.