It is not possible to default a single customer to proforma required. There is a control flag called PROFORMA_ORDER_DEFAULT, but this defaults all the customers to proforma required, not just a particular one. If you do wish to activate this control flag, you will need to follow these steps:
Go to Options.
Select System Maintenance.
Click on System Settings.
Go to GENERAL System Settings.
Select Control File.
Search for PROFORMA_ORDER_DEFAULT in the Control Item field.
Go into Modify mode and change its value from No to Yes.
Click on Exit.
Enabling this control flag will default all the customers to a proforma required.