AFAIK you can upgrade someone else on a saver Y ticket once the online checkin opens . This window is valid till 6 hours before the flight.(telephone /online)

In the last 6 hours before the flight you have 2 options

1.Upgrade at the check in desk at the airport
2. On board upgrade.

Obviously the member whose miles are being used has to be travelling on the same flight for option 2. I have never tried option 1 for someone else. The rules dont seem to be clear on this one.