Peter
To exclude those customers who have already been selected, you can use an XPath constraint on a Datagrid, Dataview, TemplateGrid, etc. For instance, if the association between Customer and OrderStatus entities is called Customer_OrderStatus, your xpath would look like this:
not(Customer_OrderStatus/OrderStatus)
This will return all customers who are not associated with an Order Status.
The popup can be accomplished with an onChange microflow on the appropriate field on your form OR an EventHandler microflow on the appropriate entity. One of the Action types in a microflow is to display a message.
Mike
The entities are called Customer and OrderStatus and the association is called CustomerOrderStatus when i go to the datagrid of customerselect (the page the reference set selector selects its customers from) and at properties -> Data source -> xpath constraint i put in your not(Customer_OrderStatus/OrderStatus) it says: required (...)+ loop did not match anything at input 'not'.