Hi Julian,
You could add a custom field to a users profile page and allow them to tick a checkbox if they have completed a first aid course. You can then use this to create an audience.
Alternatively if you only need to do this for one course, you could add a page/label activity and set activity completion for when users manually mark the activity as complete. You can then restrict access to subsequent activities in the course based on weather or not users have marked the initial activity as complete.