form_condition.html

...
  {% if form.can_edit_captain_field() %}
    <input type=checkbox name=is_captain id=id_is_captain {% if object.is_captain %}checked{% endif %}>
  {% else %}
...

form_condition.py

class PirateForm(ModelForm):
    def __init__(...):
        self.user = user
        ...
        if not self.can_edit_captain_field():
            del self.fields["is_captain"]
   
    def can_edit_captain_field(self):
        # NB: this condition is used both in the form object's __init__() method
        # above and in the template. This is critical that "self.fields" stays in
        # sync with the fields displayed in the HTML form, else it might cause
        # nasty bugs where fields are silently reset. See BUG-1234.
        return bool(self.user)