Which drupal function can you use to invoke a function stored in contrib module (e.g. in an .inc file)?
You can use module_load_include

  // Load node.admin.inc from the node module.
  module_load_include('inc', 'node', 'node.admin');
  // Load content_types.inc from the node module.
  module_load_include('inc', 'node', 'content_types');


Then you will be able to call the function that is in those files.