I have used my PMUs to upgrade friends flying without me twice before.
As long as they are in the right fare class, neither the medallion phone agent nor the person at the airport counter when I handed in the paper cert ever cared. In both cases my friends flew F w/o issue.
FWIW these were both domestic flights. I've never tried for international. Also, I always went to an airport counter a few days before my friends' flights and handed in the signed PMU myself.