Add an extra table in your access db (dbtables) with user and db tablenames as headers. Fill in the tablesnames field with all the names of the tables in you db. Now each time an update button is pressed on your application, it should first search the dbtables table for a match to the current table you wish to update and so display your message with the last user who updated the form. In the code, you update the dbtable table by filling in the user field against the corresponding db table field. The user field can be populated by the login form I presume you have.
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