Albin, I'm not sure if I understood what you are trying to do. Aren't the "Customer" file related with the "Order" file?
Anyway, if you use the "Set File_field_changed_value", you don't need to change nothing else. It will change the value in the DEO, and change the DDO's value in buffer and set the DDO's "File_Field_changed_state" to true. So, the DDO will save the changes. If you use the "Set File_Field_Current_Value", only the DEO and buffer value will be updated, but no changes in the "File_Field_Changed_State", and, if you save other changes in the view, this will be discarded.
Sorry if I misunderstood your question, and, if I said anything wrong, someone (with a better English
) correct me, please.