As a CE pax, you're not entitled to a guest, and the
Qantas Club benefits indicate that you have to be on the same flight:
Invite a guest
When visiting an international Qantas Club lounge, you are able to invite one guest each time you visit. Both you and your guest need to be travelling together further that day on a Qantas or British Airways flight. Platinum Frequent Flyers and their guest are welcome in the lounge, even when not travelling on Qantas or British Airways.
Looks like your colleague is out of luck.