guess the regular way would be to use the Control Panel for my friend’s enveloping site to create a database, and then add the details of that DB to the wp-config.php file that’s part of my WP install in the folder.

Correct

I worry that a database created in my friend’s Control Panel might effect something in my friend’s site.

If you’re creating a new database, no.

I checked into the link you gave, and Flywheel sounded interesting. But given that the day was seeming long, I decided to let Mor10 from lynda.com talk me through doing a local install using Bitnami. Went well AFAIK. Time for my recliner and a non-alcohol beer. And PBS.