On a different PNR, one can log on own Skywards account and 'manage booking' for a 3rd party - even if not flying anywhere / not same flight.. This is possible as showed above just by knowing booking reference and surname. If an 'upgrade with miles' is possible on that specific fare and available for that specific flight then it can be done online (otherwise if any flight changes have to done, only the original requestor or by travel agency if 1st segment when booked by one can do it).

Now with introduction of more restrictive EK fares, situation is a bit different - meaning i.e., Special and Saver only at Airport upgrade etc, this is bit more complicated...

As example, once my gf was flying to LAX (her company book her EK Eco Saver fare) and the only thing i could think off to upgrade her was to be on 'same flight as her' just to do her upgrade with miles even without plans to fly.. (other option could had been to buy her miles own Acc or transfer to her with a cost which i didnt want) - so i end up buying my own ticket Flex Plus with 'friendly refund' policy and upgrade her at airport check-in using my miles as i was still PAX same flight right before my cancelation . We were lucky since many J seats were available hence no need to wait for check-in to close, otherwise my own ticket would had turn in a ' No Show' (cancelation has a min time window prior to ETD) with different refund policy ... It worked well that time since got my refund later and she got J A380 with my miles.. :-)