Hi..yes you are at the right place..extended notification is the solution.
Top of this, if user wants to act without login to SAP and being in offline mode..that is also possible.
Please check below links which may help you..
Offline Workflow Approval in ECC R/3 without SAP Logon from E-Mail (Outlook)
Offline/External Mail Approval Process without using SAP Part-4
BR
Ansumesh