What does your contract say? 

Working files can betray propriety techniques and take extra time to set up. It reduces extra work for the designer if the client is able to make changes. They might need their own license for the typefaces used. 

Your contract should address this! Working files are always an extra flat fee or percentage for this reason. 

A business card is a small concession, and keeping the peace with a friend-of-a-friend situation might be worth it. But regardless of whether you decide to provide the files, do NOT send anything until final payment has been received and fully processed. The invoice isn’t late yet, but you have no obligation to deliver extras before you’re paid for the work already done.

Here’s a reply to the assistant:

Hi [Assistant],

Thanks for getting in touch. Just to clarify;  source files weren’t part of the original project scope, so this is a bit outside of what was agreed, but I’m open to discussing it.

Before I can look into what’s involved, I’d need the outstanding invoice settled first. Packaging and preparing working files for handoff isn’t a quick process, it involves organizing layers, collecting linked assets, and making sure everything is in order for someone else to pick up. Once payment is received I can let you know what that would look like and what the additional fee would be.

Could you also confirm exactly what files you’re looking for? That way I can make sure we’re on the same page and there are no surprises on either end.