Main note

With standard (non-headless/non-decoupled) version of Drupal the structure of the pages is highly dependent on the Drupal developer so be sure to learn the frontend basics thoroughly to understand the front-end developer's needs and decrease development costs.

Theming

The main work of front-end developer in theme consists of styling, templating and scripting: Drupal's theme hooks is more about work with PHP and what comes from the backend side so please be sure to provide front-end developer with variables for templates and don't ask the frontend developer to work with the hooks if he/she not interested to evolve into Drupal developer, at least at the beginning of learning of Drupal.

Additional

Also, be sure to check Drupal Good Practices