Hi, Lauren.
/wp-content/plugins/the-events-calendar-filterbar/src/Tribe/Filter.php does not support the template override system (see ourĀ Themer’s Guide)
Only files in the “views” folder are (this is true for all our plugins), such as /wp-content/plugins/the-events-calendar-filterbar/src/views/
However, you might be able to leverage the PHP classes to ultimately get what you want.
Or, if you have suggestions for improving our code, we might be able to get you access to the GitHub repository to be able to submit a pull requests (or if just a minor change in a single file, I could submit it for you).
Please let me know if you have any follow-up questions on this topic.