form_with_comment.html

  {# Keep this condition in sync with PirateForm #}
  {% if user.name == "Alice" %}
    <input type=checkbox name=is_captain id=id_is_captain {% if object.is_captain %}checked{% endif %}>
  {% else %}

form_with_comment.py

class PirateForm(ModelForm):
  ...
   def __init__(...):
        # fields restriction
        # should be kept in sync with the template form.html
        if not user:
            del self.fields["is_captain"]