You cannot lock the subfile record unless you locked all the records in the subfile.
But, you when a user selects a record, you can lock the DB record that is associated with the record.
You would do this by doing a chain and having the file identified as an Update File.
This is not a good solution as the user could walk away fromn this task and hte record would stay locked.
Generally, you take care of this programmatically..
When you write the subfile record, include a hidden fields of the database record.
Then when someone tries to do an update, compare the database record with the values in the hidden fields. If they do not match, someone else updated the record. nad you can display a message.