Pop up user note when customer selected in sales order via Workflow

One option with no coding would be to have a non-stored custom field on the sales order that has source list=entity and source from=customer info.

The alert can be done with a workflow. Below are the steps on how to create the workflow that will produce an alert when the client field is edited.
1. Navigate to Customization > Workflow > Workflows > New.
    Record Type = Transaction, Sub Types = Sales Order
    Event Definition = On Create, On Update
    Trigger Type = Before Record Load
2. Create a State.
3. Add a 'Show Message' action.
    Trigger On = After Field Sourcing
    Client Fields = Entity
    Parameters: Text = {custbody8} (name of the field enclosed with curly braces)