Ok I might misunderstand you, but is my assumption right that you have data in your Mendix application that is wrong and needs to be set right? Because then I just would write a hotfix microflow that retrieves those objects and do the corrections in that microflow. I would indeed not fix this through SQL statements directly on the database.
Since your client refuses the SQL-method and defines the requirement to change the data from ‘the application itself’ also will prevent the client from accepting any other way of writing to the database. So, create a page showing the errorneous data and on that page aloow the customer to correct the data and save it.
Also, prevent the data from being faulty. Correct the process that causes this problem.