When a module has a composer dependency, should it be automatically installed when the module is installed? Trying to resolve a debate - if a drupal site that is configured correctly installs a module with a composer dependency, should I expect the composer dependency to be installed automatically during the install?

The other person thinks that modules with composer dependencies have to be installed through command line to work, I think that if things were configured correctly it should happen on its own. 

Does someone know how it should work?
If you install the module with composer, yes, dependencies will be fetched. If you download the tar and put it in the site, it will not.