If the buyer is telling the truth, that's exactly what the postie said he/she did, though I'd not use the term 'forging a signature' as no where has that been said - he could sign it with a simple line as there's no requirement for the signature for RMSD to be that of the named recipient (evidently as shown by many here saying that get others to sign for their packages). It's not exactly unknown behaviour for a postie to sign for a package.
Also, out of interest I thought message boards and forums came under slander rather than libel due to their conversational nature? Not that slander is acceptable either!
It'll be pretty hard to ID the postie. There's also no benefit to identifying him/her whatsoever. No reason to think the postie was doing anything but try and be helpful so the recipient didn't have to go to the depot.