Pretty much like at the interview, people claim they had to show it, yet I don't know anyone who needed it at my office despite them listing it on the white board.
But your friend will need it for your old age pension or similar, so having one available is a good idea. And if they DO get asked for one, here's a copy and the original (or VOS) is in the mail due to x is a lot better than "oh I don't have it is a copy good enough".
Just an update, despite what the letters claim NO-ONE I saw at the Oath was asked for their CoPR (I saw that other people had), the guy next to me confirmed my experience and observations that as long as you have the invite letter, signed the photos may be taken part (page 3), and your PR card(s) to surrender nothing else was required. In fact I saw people handing over more than this stuff with the letter, and they were given it all back saying not needed.
That said, the Judge confirmed you should keep it for things later in life where it could be requested (OAP/OAS is the one I know gets mentioned often here).