Known vulnerabilities for when a guest executes a stored procedure on SQL Server
I would first question why they need direct access to the database. You might ask your manager or legal department if the security policy for the company allows granting this type of access. Is it really needed if they are just going to execute a stored procedure on a regular basis.

As you stated this is going to be on a regular basis I would setup an SSIS package to export the information to a file. Then either have the package email the file (if not to large) or put it on a UNC directory for them to come and download themselves. If they want to determine when the data is pulled down setup a script for them to execute outside of SQL Server that simply runs the package.