Hi,
One way or another, they will need a SAP logon. When the workitem is approved, it has to store in the workflow log the userid of the actual approver.
I would not recommend creating a generic userid for remote approvals (as discussed in the link above). Emails can be (auto) forwarded to anyone, and this gives the recipient the power to approve and reject big-ticket items in a Production system, in the CEO's name! You could lose your job over a design like that 
I get requests for this kind of functionality all the time. It's your job as a consultant to push back!
cheers
Paul