We have a SharePoint site within our department where people outside the department do not have access. However due to the nature of our work, we require responses to our work from people outside the department. Do they need site access to be able to view and approve (with comments) the workflow item?
If the Tasks lives in SharePoint site and user does not have access to SharePoint site, they cannot approve the task.
Nintex Workflow Cloud provides "Task Approval" mechanism, you can try using that one.
Hope this helps
Hope this works.