Hello Joanna

Thanks for posting in the Community!

You are correct - your client will need to create an account to be able to access the document you shared with them and edit it. They will however not need to have a paid subscription.

It is important to note that if someone from a higher level account shares a document using paid features with a lower level account the lower level account will have limited or no access to those paid features.

The following page provides an overview of what feature is accessible at each subscription level: Lucidchart plans

Let me know if you have any questions!