If it is all on one ticket, then Jetstar will check through ( as per
https://www.jetstar.com/au/en/help/a...gage-transfers )
If they are separate bookings, then you are not connecting, but just doing 1 jounrey from ZQN-MEL and a separate journey from MEL-LHR; in this case you will have to clear immigration and customs and then head to Qantas check in desk to check bags in for the onward flight; if there is a problem with the JQ flight and she misses the QF checkin closure time, then she will be just a no-show and have to change the MEL-LHR ticket accordingly , so make sure to have left plenty of time