Requirement:
If there is a requirement where each row of timesheet needs to be Billed individually along with the Entire timesheet billing Status, the below given solution will work.
Solution:
Create a Row Level User Defined Field and let it have a name "RowBilling". Provide two parameters in this UDF – 'Yes' and 'No' and make the value 'No' as the Default. This will take care of the Timesheet Row Billing Requirement.
Timesheet already provides a Billed Column under the Historical Timesheet section where HR Person/Finance can mark the entire Timesheet as 'Billed'.
With the above given options, we can create a custom column in Timesheet Detail report to show any timesheet row if that has been billed or not. To do this, use the custom formula given below and that will provide the Billing Status of each row of timesheet for all users.

  Formula :

     If (RowBilling = "Yes", "Yes", If ( RowBilling = "No"  & BilledStatus = "Yes", "Yes", "No"))

  The above formula checks if the column 'RowBilling' has a value 'Yes' to it, if it does, then the Billing Status is Yes, if no, then it again checks if the 'RowBilling' column shows 'No', if it does, then it check if Billed Status column shows 'Yes', if it does, then the result 'Yes' is displayed, or else the value 'No' is displayed.